helpful-variable crashes for variables defined on C-source about helpful HOT 9 CLOSED

Debugger entered--Lisp error: (wrong-type-argument stringp C-source)
  string-match("\\`src/\\(.*\\.\\(c\\|m\\)\\)\\'" C-source)
  #f(compiled-function (symbol type library) "Search for SYMBOL's definition of type TYPE in LIBRARY.\nVisit the library in a buffer, and return a cons cell (BUFFER . POSITION),\nor just (BUFFER . nil) if the definition can't be found in the file.\n\nIf TYPE is nil, look for a function definition.\nOtherwise, TYPE specifies the kind of definition,\nand it is interpreted via `find-function-regexp-alist'.\nThe search is done in the source for library LIBRARY." #<bytecode 0x196289f>)(system-type defvar C-source)
  ad-Advice-find-function-search-for-symbol(#f(compiled-function (symbol type library) "Search for SYMBOL's definition of type TYPE in LIBRARY.\nVisit the library in a buffer, and return a cons cell (BUFFER . POSITION),\nor just (BUFFER . nil) if the definition can't be found in the file.\n\nIf TYPE is nil, look for a function definition.\nOtherwise, TYPE specifies the kind of definition,\nand it is interpreted via `find-function-regexp-alist'.\nThe search is done in the source for library LIBRARY." #<bytecode 0x196289f>) system-type defvar C-source)
  apply(ad-Advice-find-function-search-for-symbol #f(compiled-function (symbol type library) "Search for SYMBOL's definition of type TYPE in LIBRARY.\nVisit the library in a buffer, and return a cons cell (BUFFER . POSITION),\nor just (BUFFER . nil) if the definition can't be found in the file.\n\nIf TYPE is nil, look for a function definition.\nOtherwise, TYPE specifies the kind of definition,\nand it is interpreted via `find-function-regexp-alist'.\nThe search is done in the source for library LIBRARY." #<bytecode 0x196289f>) (system-type defvar C-source))
  find-function-search-for-symbol(system-type defvar C-source)
  find-variable-noselect(system-type C-source)
  helpful--definition(system-type nil)
  helpful--source(system-type nil)
  helpful-update()
  helpful-variable(system-type)
  funcall-interactively(helpful-variable system-type)
  #<subr call-interactively>(helpful-variable nil nil)
  apply(#<subr call-interactively> helpful-variable (nil nil))
  call-interactively@ido-cr+-record-current-command(#<subr call-interactively> helpful-variable nil nil)
  apply(call-interactively@ido-cr+-record-current-command #<subr call-interactively> (helpful-variable nil nil))
  call-interactively(helpful-variable nil nil)
  command-execute(helpful-variable)