diff --git a/input/regression/beam-quarter.ly b/input/regression/beam-quarter.ly
index 7c9431de28d8958f70e7a3da19e417c7a0fcdc2a..8717947b42b826c2c1854d5acdfcbea5931c4b8c 100644
--- a/input/regression/beam-quarter.ly
+++ b/input/regression/beam-quarter.ly
@@ -1,6 +1,6 @@
 \version "2.19.21"
 
-#(ly:set-option 'warning-as-error #f)
+#(ly:set-option 'warning-as-error #t)
 #(ly:expect-warning (_ "stem does not fit in beam"))
 #(ly:expect-warning (_ "beam was started here"))
 
diff --git a/input/regression/bom-mark.ly b/input/regression/bom-mark.ly
index 31f06b7ab6632905c996296178f0086abed75e1c..2817bc4349ed80ad092eb2e369ccae50691a9c8d 100644
--- a/input/regression/bom-mark.ly
+++ b/input/regression/bom-mark.ly
@@ -1,5 +1,5 @@
 #(ly:expect-warning (_ "stray UTF-8 BOM encountered"))
-#(ly:set-option 'warning-as-error #f)
+#(ly:set-option 'warning-as-error #t)
  \version "2.16.0"
 
 
diff --git a/input/regression/clef-warn.ly b/input/regression/clef-warn.ly
index 7814e757259ba18d288017fc9d31f0f6d483e33d..62eaffab59fbd25f5c3cd223efc76fc6438052f5 100644
--- a/input/regression/clef-warn.ly
+++ b/input/regression/clef-warn.ly
@@ -1,6 +1,6 @@
 \version "2.16.0"
 
-#(ly:set-option 'warning-as-error #f)
+#(ly:set-option 'warning-as-error #t)
 #(ly:expect-warning (_ "unknown clef type `~a'") "foo")
 % We don't need to match the whole list of clefs, the beginning will suffice
 #(ly:expect-warning (_ "supported clefs: ~a") "C F G G2")
diff --git a/input/regression/glissando-no-break.ly b/input/regression/glissando-no-break.ly
index 713bc0514f486e5a8bed44e88008e1896074622f..7e3dcbe8da6568b450151400311a6e3bd69e3662 100644
--- a/input/regression/glissando-no-break.ly
+++ b/input/regression/glissando-no-break.ly
@@ -1,5 +1,5 @@
 \version "2.19.21"
-#(ly:set-option 'warning-as-error #f)
+#(ly:set-option 'warning-as-error #t)
 #(ly:expect-warning (_ "forced break was overridden by some other event, should you be using bar checks?"))
 
 \header {
diff --git a/input/regression/harp-pedals-sanity-checks.ly b/input/regression/harp-pedals-sanity-checks.ly
index 6a2e3cb6f21b85087ea7e1b78c8af69bb6940e57..b83eb55853e7d003e2ce6222fc44f0cc5a259c85 100644
--- a/input/regression/harp-pedals-sanity-checks.ly
+++ b/input/regression/harp-pedals-sanity-checks.ly
@@ -1,6 +1,6 @@
 \version "2.21.0"
 
-#(ly:set-option 'warning-as-error #f)
+#(ly:set-option 'warning-as-error #t)
 #(ly:expect-warning (_ "Harp pedal diagram contains dividers at positions ~a.  Normally, there is only one divider after the third pedal.") '(1 3 5))
 #(ly:expect-warning (_ "Harp pedal diagram contains dividers at positions ~a.  Normally, there is only one divider after the third pedal.") '(4))
 #(ly:expect-warning (_ "Harp pedal diagram contains ~a pedals rather than the usual 7.") 5)
diff --git a/input/regression/harp-pedals.ly b/input/regression/harp-pedals.ly
index ebbe899342d9a1dd940cf6496beb572a478f464c..39023a305bc3b1de768aa9118c9555352665585a 100644
--- a/input/regression/harp-pedals.ly
+++ b/input/regression/harp-pedals.ly
@@ -1,6 +1,6 @@
 \version "2.21.0"
 
-#(ly:set-option 'warning-as-error #f)
+#(ly:set-option 'warning-as-error #t)
 #(ly:expect-warning (_ "Harp pedal diagram contains ~a pedals rather than the usual 7.") 0)
 #(ly:expect-warning (_ "Harp pedal diagram does not contain a divider (usually after third pedal)."))
 #(ly:expect-warning (_ "Harp pedal diagram contains ~a pedals rather than the usual 7.") 1)
diff --git a/input/regression/header-cyclic-reference.ly b/input/regression/header-cyclic-reference.ly
index 765f57091d41e40ce7a43c27f26a2851d84bce32..2a50d762630b44a7ce9884d6cb19986adfe0cdca 100644
--- a/input/regression/header-cyclic-reference.ly
+++ b/input/regression/header-cyclic-reference.ly
@@ -1,5 +1,5 @@
 \version "2.16.0"
-#(ly:set-option 'warning-as-error #f)
+#(ly:set-option 'warning-as-error #t)
 #(ly:expect-warning (_ "Recursive definition of property ~a detected!") 'header:title)
 #(ly:expect-warning (_ "Recursive definition of property ~a detected!") 'header:composer)
 
diff --git a/input/regression/incompatible-stem-warning.ly b/input/regression/incompatible-stem-warning.ly
index 7115741b1aa98daa5df59fd2d7cdae08d550ed13..ce39ab50d691b95c045c27e6e168bcb8ff42a1cb 100644
--- a/input/regression/incompatible-stem-warning.ly
+++ b/input/regression/incompatible-stem-warning.ly
@@ -1,5 +1,5 @@
 \version "2.16.0"
-#(ly:set-option 'warning-as-error #f)
+#(ly:set-option 'warning-as-error #t)
 #(ly:expect-warning (ly:translate-cpp-warning-scheme "adding note head to incompatible stem (type = %d/%d)") 1 1)
 #(ly:expect-warning (ly:translate-cpp-warning-scheme "maybe input should specify polyphonic voices"))
 #(ly:expect-warning (ly:translate-cpp-warning-scheme "adding note head to incompatible stem (type = %d/%d)") 1 8)
diff --git a/input/regression/instrument-switch-invalid-warning.ly b/input/regression/instrument-switch-invalid-warning.ly
index 0ef544cc3d60d474cda1ebd42963f1b1a028c32a..97919042e1d154324ecd8a2f8d3d8d447c0e3c7a 100644
--- a/input/regression/instrument-switch-invalid-warning.ly
+++ b/input/regression/instrument-switch-invalid-warning.ly
@@ -1,5 +1,5 @@
 \version "2.19.21"
-#(ly:set-option 'warning-as-error #f)
+#(ly:set-option 'warning-as-error #t)
 #(ly:expect-warning (_ "No such instrument: ~a") "bassClar")
 
 \header {
diff --git a/input/regression/invalid-engraver.ly b/input/regression/invalid-engraver.ly
index b83befbae28ec1e8817e6179a65310ab99cde05d..351daadd352aee3ae91335ab8698182b41213fbd 100644
--- a/input/regression/invalid-engraver.ly
+++ b/input/regression/invalid-engraver.ly
@@ -1,6 +1,6 @@
 \version "2.16.0"
 
-#(ly:set-option 'warning-as-error #f)
+#(ly:set-option 'warning-as-error #t)
 #(ly:expect-warning (ly:translate-cpp-warning-scheme "unknown translator: `%s'") "Rhythmic_column_engraver_foo")
 #(ly:expect-warning (ly:translate-cpp-warning-scheme "cannot find: `%s'") "Rhythmic_column_engraver_foo")
 
diff --git a/input/regression/loglevels.ly b/input/regression/loglevels.ly
index 91b239a3e0c6309963d9b3ff99f7152689ccc2d9..e37530a69934e30b3c9d0ed01b4db60c234fe2c1 100644
--- a/input/regression/loglevels.ly
+++ b/input/regression/loglevels.ly
@@ -1,6 +1,6 @@
 \version "2.19.22"
 
-#(ly:set-option 'warning-as-error #f)
+#(ly:set-option 'warning-as-error #t)
 #(ly:expect-warning (_ "Test warning\n"))
 #(ly:expect-warning (_ "already have slur"))
 
diff --git a/input/regression/markup-brace-warning.ly b/input/regression/markup-brace-warning.ly
index cf8ef8dc91cdd88998548e6b87016a53c9c6470b..20b3c1f15d6292d284f5520b2df3a2b3d3bc4b61 100644
--- a/input/regression/markup-brace-warning.ly
+++ b/input/regression/markup-brace-warning.ly
@@ -1,6 +1,6 @@
 \version "2.16.0"
 
-#(ly:set-option 'warning-as-error #f)
+#(ly:set-option 'warning-as-error #t)
 #(ly:expect-warning (_ "no brace found for point size ~S ") 10)
 #(ly:expect-warning (_ "defaulting to ~S pt") 10.5)
 
diff --git a/input/regression/markup-cyclic-reference.ly b/input/regression/markup-cyclic-reference.ly
index c0a952e3ea2ceb9b312f2df97e1fb53a340643be..45292d489d62c984cbece24e0951b541133b1e72 100644
--- a/input/regression/markup-cyclic-reference.ly
+++ b/input/regression/markup-cyclic-reference.ly
@@ -1,5 +1,5 @@
 \version "2.19.13"
-#(ly:set-option 'warning-as-error #f)
+#(ly:set-option 'warning-as-error #t)
 #(ly:expect-warning (ly:translate-cpp-warning-scheme "Markup depth exceeds maximal value of %d; Markup: %s") 1024 'cycle-markup)
 #(ly:expect-warning (ly:translate-cpp-warning-scheme "Markup depth exceeds maximal value of %d; Markup: %s") 1024 'cycleII-markup)
 
diff --git a/input/regression/markup-depth-non-terminating.ly b/input/regression/markup-depth-non-terminating.ly
index 76db67f19369a20e607b8dbcfab104cc37cee2ef..a19d1e240e11f2af368b5b3578a8372a44285b18 100644
--- a/input/regression/markup-depth-non-terminating.ly
+++ b/input/regression/markup-depth-non-terminating.ly
@@ -1,5 +1,5 @@
 \version "2.16.0"
-#(ly:set-option 'warning-as-error #f)
+#(ly:set-option 'warning-as-error #t)
 #(ly:expect-warning (ly:translate-cpp-warning-scheme "Markup depth exceeds maximal value of %d; Markup: %s") 1024 "recursive-explosion-markup")
 
 \header {
diff --git a/input/regression/markup-music-glyph.ly b/input/regression/markup-music-glyph.ly
index 32c921b98a4af140daa98a2f0bf2c5e29f830524..c5bfdfa083303d4a053a729d9fdffead21ddd889 100644
--- a/input/regression/markup-music-glyph.ly
+++ b/input/regression/markup-music-glyph.ly
@@ -1,6 +1,6 @@
 \version "2.21.0"
 
-#(ly:set-option 'warning-as-error #f)
+#(ly:set-option 'warning-as-error #t)
 #(ly:expect-warning (_ "Cannot find glyph ~a") "UNKNOWN-GLYPH")
 
 \header {
diff --git a/input/regression/page-break-warn-forbidden.ly b/input/regression/page-break-warn-forbidden.ly
index 514962e1fc2c7c165ebd3c978e0d617e527b098f..f485cd57b6e0883a2e9bbee5559882bafddc008a 100644
--- a/input/regression/page-break-warn-forbidden.ly
+++ b/input/regression/page-break-warn-forbidden.ly
@@ -1,6 +1,6 @@
 \version "2.16.0"
 
-#(ly:set-option 'warning-as-error #f)
+#(ly:set-option 'warning-as-error #t)
 #(ly:expect-warning (_ "forced break was overridden by some other event, should you be using bar checks?"))
 #(ly:expect-warning (_ "forced break was overridden by some other event, should you be using bar checks?"))
 
diff --git a/input/regression/page-spacing-system-count-overfull.ly b/input/regression/page-spacing-system-count-overfull.ly
index 32c744fb844b1aee700a7745d0a42b983ab89df6..be02cc6b405356dac37825194bfba09c16ed1c73 100644
--- a/input/regression/page-spacing-system-count-overfull.ly
+++ b/input/regression/page-spacing-system-count-overfull.ly
@@ -1,5 +1,5 @@
 \version "2.16.0"
-#(ly:set-option 'warning-as-error #f)
+#(ly:set-option 'warning-as-error #t)
 #(ly:expect-warning (_ "cannot find line breaking that satisfies constraints"))
 
 \header {
diff --git a/input/regression/page-turn-page-breaking-badturns.ly b/input/regression/page-turn-page-breaking-badturns.ly
index 52838112c7b89fa926cd80ef76dd9e72fe3cc8d8..327e8814ad90d1bc69f17fdff81eebda22411ff4 100644
--- a/input/regression/page-turn-page-breaking-badturns.ly
+++ b/input/regression/page-turn-page-breaking-badturns.ly
@@ -1,5 +1,5 @@
 \version "2.19.21"
-#(ly:set-option 'warning-as-error #f)
+#(ly:set-option 'warning-as-error #t)
 #(ly:expect-warning (_ "cannot fit the first page turn onto a single page.  Consider setting first-page-number to an even number."))
 
 \header {
diff --git a/input/regression/paper-margins-consistency.ly b/input/regression/paper-margins-consistency.ly
index 91a13d5468aa19ce910b0e31e89a623a304c2490..4048355462c169dacff2d9cfbb77baf31d4e4c20 100644
--- a/input/regression/paper-margins-consistency.ly
+++ b/input/regression/paper-margins-consistency.ly
@@ -1,6 +1,6 @@
 \version "2.19.21"
 
-#(ly:set-option 'warning-as-error #f)
+#(ly:set-option 'warning-as-error #t)
 #(ly:expect-warning (_ "margins do not fit with line-width, setting default values"))
 
 \header {
diff --git a/input/regression/paper-margins-overrun.ly b/input/regression/paper-margins-overrun.ly
index 6c929233e674fd6eabb72430402fb9dd5afb91e7..e2054ceda8caa2bd90f5d913c2e5392e86a0ef36 100644
--- a/input/regression/paper-margins-overrun.ly
+++ b/input/regression/paper-margins-overrun.ly
@@ -1,6 +1,6 @@
 \version "2.19.21"
 
-#(ly:set-option 'warning-as-error #f)
+#(ly:set-option 'warning-as-error #t)
 #(ly:expect-warning (_ "systems run off the page due to improper paper settings, setting default values"))
 
 \header {
diff --git a/input/regression/phrasing-slur-multiple.ly b/input/regression/phrasing-slur-multiple.ly
index adf8221845b9a8077eca0c446bb161607f51aee3..ae3277e14fb79efc27f5d5e5fd773ca87a7cf87e 100644
--- a/input/regression/phrasing-slur-multiple.ly
+++ b/input/regression/phrasing-slur-multiple.ly
@@ -1,6 +1,6 @@
 \version "2.19.29"
 
-#(ly:set-option 'warning-as-error #f)
+#(ly:set-option 'warning-as-error #t)
 #(ly:expect-warning (_ "already have phrasing slur"))
 #(ly:expect-warning (_ "cannot end phrasing slur"))
 #(ly:expect-warning (_ "unterminated phrasing slur"))
diff --git a/input/regression/skiptypesetting-bar-check.ly b/input/regression/skiptypesetting-bar-check.ly
index ec467da75a2d574e623bb856748613b4329dcae2..5e3cadc7ff90a89ed45cbd431bc70d408aafb93c 100644
--- a/input/regression/skiptypesetting-bar-check.ly
+++ b/input/regression/skiptypesetting-bar-check.ly
@@ -1,5 +1,5 @@
 \version "2.19.21"
-#(ly:set-option 'warning-as-error #f)
+#(ly:set-option 'warning-as-error #t)
 #(ly:expect-warning (ly:translate-cpp-warning-scheme "barcheck failed at: %s") "3/4")
 
 \header {
diff --git a/input/regression/slur-multiple-linebreak.ly b/input/regression/slur-multiple-linebreak.ly
index 206f488244e884edd22df033167f930756c0fd87..b3d1011fba5b65ac964a9612e1fd194ca87accb0 100644
--- a/input/regression/slur-multiple-linebreak.ly
+++ b/input/regression/slur-multiple-linebreak.ly
@@ -1,6 +1,6 @@
 \version "2.19.21"
 
-#(ly:set-option 'warning-as-error #f)
+#(ly:set-option 'warning-as-error #t)
 #(ly:expect-warning (_ "already have slur"))
 
 \header {
diff --git a/input/regression/slur-multiple.ly b/input/regression/slur-multiple.ly
index 007b69773ec293a7d30af9d6043bda472b3fde3b..15ac72000af3c0a9026d4fe9927f40fea4a4fc2d 100644
--- a/input/regression/slur-multiple.ly
+++ b/input/regression/slur-multiple.ly
@@ -1,6 +1,6 @@
 \version "2.19.29"
 
-#(ly:set-option 'warning-as-error #f)
+#(ly:set-option 'warning-as-error #t)
 #(ly:expect-warning (_ "already have slur"))
 #(ly:expect-warning (_ "cannot end slur"))
 #(ly:expect-warning (_ "unterminated slur"))
diff --git a/input/regression/stem-tremolo.ly b/input/regression/stem-tremolo.ly
index 032f900ecc0a32b1f2a7c4fb6a969c8e9a451fb3..3631b4912cc13affd14d88343db6e60c64d8ec96 100644
--- a/input/regression/stem-tremolo.ly
+++ b/input/regression/stem-tremolo.ly
@@ -1,6 +1,6 @@
 \version "2.17.6"
 
-#(ly:set-option 'warning-as-error #f)
+#(ly:set-option 'warning-as-error #t)
 #(ly:expect-warning (_ "tremolo duration is too long"))
 #(ly:expect-warning (_ "tremolo duration is too long"))
 
diff --git a/input/regression/tablature-micro-tone.ly b/input/regression/tablature-micro-tone.ly
index a7f891c048ff65f8a97384c80b8f2caf15c24269..28ee716588a1003a720531fce500f84727bab769 100644
--- a/input/regression/tablature-micro-tone.ly
+++ b/input/regression/tablature-micro-tone.ly
@@ -12,7 +12,7 @@ case for the last four of the examples pitches.
 Micro-tones assigned to strings work nicely."
 }
 
-#(ly:set-option 'warning-as-error #f)
+#(ly:set-option 'warning-as-error #t)
 #(for-each
   (lambda (pitch)
     (ly:expect-warning (_ "No string for pitch "))
diff --git a/input/regression/tablature-negative-fret.ly b/input/regression/tablature-negative-fret.ly
index 28d08f427dcf289d638d9ac664a173edf974af58..cd0ebf5f8cc6b7ce2b03482f32a72f35f65b9dd2 100644
--- a/input/regression/tablature-negative-fret.ly
+++ b/input/regression/tablature-negative-fret.ly
@@ -1,5 +1,5 @@
 \version "2.19.21"
-#(ly:set-option 'warning-as-error #f)
+#(ly:set-option 'warning-as-error #t)
 #(ly:expect-warning (_ "Requested string for pitch requires negative fret: string ~a pitch ~a") 1 "#<Pitch c' >")
 #(ly:expect-warning (_ "Ignoring string request and recalculating."))
 #(ly:expect-warning (_ "Negative fret for pitch ~a on string ~a") "#<Pitch c' >" 1)
diff --git a/input/regression/tablature-zero-finger.ly b/input/regression/tablature-zero-finger.ly
index a22a58e51ebe32f253e8585145c0f5a2f30135cb..f9ffc4d0f8b2f39355c1671940e65581c0f28fee 100644
--- a/input/regression/tablature-zero-finger.ly
+++ b/input/regression/tablature-zero-finger.ly
@@ -1,5 +1,5 @@
 \version "2.19.21"
-#(ly:set-option 'warning-as-error #f)
+#(ly:set-option 'warning-as-error #t)
 #(ly:expect-warning (_ "No open string for pitch ~a") "#<Pitch f >")
 
 \header {
diff --git a/input/regression/tie-arpeggio.ly b/input/regression/tie-arpeggio.ly
index eabcaa78cb7779c8244ca9980d661d785810f1e8..d65333878a76ae9b52a582bf0894b7b3a198826f 100644
--- a/input/regression/tie-arpeggio.ly
+++ b/input/regression/tie-arpeggio.ly
@@ -1,5 +1,5 @@
 \version "2.19.21"
-#(ly:set-option 'warning-as-error #f)
+#(ly:set-option 'warning-as-error #t)
 #(ly:expect-warning (_ "unterminated tie"))
 #(ly:expect-warning (_ "unterminated tie"))
 
diff --git a/input/regression/tie-unterminated.ly b/input/regression/tie-unterminated.ly
index 1f5dd010afd56410e0abbb376ad6d8e43a923717..d56369a882411d79fb7a148c50ead5b07b1a5758 100644
--- a/input/regression/tie-unterminated.ly
+++ b/input/regression/tie-unterminated.ly
@@ -1,5 +1,5 @@
 \version "2.19.21"
-#(ly:set-option 'warning-as-error #f)
+#(ly:set-option 'warning-as-error #t)
 #(ly:expect-warning (_ "unterminated tie"))
 
 \header {
diff --git a/input/regression/warn-conflicting-key-signatures.ly b/input/regression/warn-conflicting-key-signatures.ly
index 12e9d575eecf538320ba13a8d322ecc293a9a4bd..48d3c5a95770116933187b6e50928e973fe7ef31 100644
--- a/input/regression/warn-conflicting-key-signatures.ly
+++ b/input/regression/warn-conflicting-key-signatures.ly
@@ -1,5 +1,5 @@
 \version "2.16.0"
-#(ly:set-option 'warning-as-error #f)
+#(ly:set-option 'warning-as-error #t)
 #(ly:expect-warning (ly:translate-cpp-warning-scheme "conflict with event: `%s'") "key-change-event")
 #(ly:expect-warning (ly:translate-cpp-warning-scheme "discarding event: `%s'") "key-change-event")
 
diff --git a/input/regression/warn-unterminated-span-dynamic.ly b/input/regression/warn-unterminated-span-dynamic.ly
index 43e0c9cbbac714739afb12b4463c8420a5179246..b77cb851991250a500ac51cfd6b928bc809f039a 100644
--- a/input/regression/warn-unterminated-span-dynamic.ly
+++ b/input/regression/warn-unterminated-span-dynamic.ly
@@ -1,6 +1,6 @@
 \version "2.16.0"
 
-#(ly:set-option 'warning-as-error #f)
+#(ly:set-option 'warning-as-error #t)
 #(ly:expect-warning (ly:translate-cpp-warning-scheme "unterminated %s") "crescendo")
 #(ly:expect-warning (ly:translate-cpp-warning-scheme "unterminated %s") "decrescendo")
 
diff --git a/lily/translator-group.cc b/lily/translator-group.cc
index 3407318246e8d3612a392d3a7d15a9d25685f651..064bbc33814a5155290b924b7f612f6665908984 100644
--- a/lily/translator-group.cc
+++ b/lily/translator-group.cc
@@ -127,6 +127,8 @@ Translator_group::create_child_translator (SCM sev)
 
   for (SCM trans : ly_scm_list (def->get_translator_names (ops)))
     {
+      SCM arg = trans; // in case we want to print the original value below
+
       if (ly_is_symbol (trans))
         trans = get_translator_creator (trans);
       if (ly_is_procedure (trans))
@@ -136,7 +138,7 @@ Translator_group::create_child_translator (SCM sev)
       Translator *instance = unsmob<Translator> (trans);
       if (!instance)
         {
-          warning (_f ("cannot find: `%s'", ly_scm_write_string (trans).c_str ()));
+          warning (_f ("cannot find: `%s'", ly_scm_write_string (arg).c_str ()));
           continue;
         }