Compile error on FreeBSD: `idn2-help.texi': No such file or directory.

Building the FreeBSD port dns/libidn2 (currently libidn2-2.3.1) fails (for me) with the following error message:

/usr/bin/make  all-am
Making all in fuzz
Making all in po
Making all in doc
--- idn2.1 ---
/bin/sh /var/ports/usr/ports/dns/libidn2/work/libidn2-2.3.1/build-aux/missing help2man --include=./idn2.h2m  --output=idn2.1 ../src/idn2
--- all ---
/usr/bin/make  all-recursive
--- all-recursive ---
Making all in reference
--- idn2--help.texi ---
../src/idn2 --help | /usr/bin/grep '^ \+-' > tmp
mv tmp idn2--help.texi
--- ./libidn2.info ---
restore=: && backupdir=".am$$" &&  am__cwd=`pwd` && CDPATH="${ZSH_VERSION+.}:" && cd . &&  rm -rf $backupdir && mkdir $backupdir &&  if (/bin/sh /var/ports/usr/ports/dns/libidn2/work/libidn2-2.3.1/build-aux/missing makeinfo --version) >/dev/null 2>&1; then  for f in ./libidn2.info ./libidn2.info-[0-9] ./libidn2.info-[0-9][0-9] ./libidn2.i[0-9] ./libidn2.i[0-9][0-9]; do  if test -f $f; then mv $f $backupdir; restore=mv; else :; fi;  done;  else :; fi &&  cd "$am__cwd";  if /bin/sh /var/ports/usr/ports/dns/libidn2/work/libidn2-2.3.1/build-aux/missing makeinfo   -I .  -o ./libidn2.info ./libidn2.texi;  then  rc=0;  CDPATH="${ZSH_VERSION+.}:" && cd .;  else  rc=$?;  CDPATH="${ZSH_VERSION+.}:" && cd . &&  $restore $backupdir/* `echo "././libidn2.info" | sed 's|[^/]*$||'`;  fi;  rm -rf $backupdir; exit $rc
./libidn2.texi:6: warning: unrecognized encoding name `utf-8'.
./libidn2.texi:489: @verbatiminclude `idn2-help.texi': No such file or directory.
makeinfo: Removing output file `./libidn2.info' due to errors; use --force to preserve.
*** [./libidn2.info] Error code 1

make[5]: stopped in /var/ports/usr/ports/dns/libidn2/work/libidn2-2.3.1/doc
1 error

The installed help2man is help2man-1.48.3

Thanks, -David