Reader error related to '# +' occurs when installing GSLL

I tried to install a package called GSLL and got the following error:

>(ql:quickload :gsll)

;;; Internal error:
;;;   ** Reader error in file #<input stream "/home/hiro/deepspace/share/deepspace/quicklisp/dists/quicklisp/software/gsll-quicklisp-eeeda841-git/polynomial.lisp" 0x55deb3e20f00>, position 2690:
;;; No dispatch function defined for character #\+
Condition of type: COMPILE-FILE-ERROR
COMPILE-FILE-ERROR while compiling #<cl-source-file "gsll" "polynomial">

When I asked a question on the GSLL mailing list, I got a reply that it couldn't handle "# +" and looked like an ECL problem.

(lisp-implementation-version)"16.1.3"
(ext:lisp-implementation-vcs-id)"f67d9a5b911f11fcad8f2b2059ad91e2a4f1e29c"
(software-type)"Linux"
(software-version)"4.19.0-6-amd64"
(machine-type)"x86_64"
*features*
(:INT64 :GSL2 :LISP-UNIT :SPLIT-SEQUENCE :CL-PPCRE :NAMED-READTABLES
 :STATIC-VECTORS :FSBV CFFI-FEATURES:FLAT-NAMESPACE CFFI-FEATURES:X86-64
 CFFI-FEATURES:UNIX :CFFI CFFI-SYS::FLAT-NAMESPACE :X86-64 :QUICKLISP
 :ASDF-PACKAGE-SYSTEM :ASDF3.1 :ASDF3 :ASDF2 :ASDF :OS-UNIX
 :NON-BASE-CHARS-EXIST-P :ASDF-UNICODE :WALKER :CDR-1 :CDR-5 :LINUX :FORMATTER
 :CDR-7 :ECL-WEAK-HASH :LITTLE-ENDIAN :ECL-READ-WRITE-LOCK :LONG-LONG :UINT64-T
 :UINT32-T :UINT16-T :COMPLEX-FLOAT :LONG-FLOAT :UNICODE :DFFI :CLOS-STREAMS
 :CMU-FORMAT :UNIX :ECL-PDE :DLOPEN :CLOS :THREADS :BOEHM-GC :ANSI-CL
 :COMMON-LISP :FLOATING-POINT-EXCEPTIONS :IEEE-FLOATING-POINT
 :PACKAGE-LOCAL-NICKNAMES :CDR-14 :PREFIXED-API :FFI :X86_64 :COMMON :ECL)