diff --git a/m4/prereq.m4 b/m4/prereq.m4
index 4c8ac9384..f478a14d9 100644
--- a/m4/prereq.m4
+++ b/m4/prereq.m4
@@ -39,7 +39,7 @@ AC_DEFUN(jm_PREREQ_CANON_HOST,
 AC_DEFUN(jm_PREREQ_QUOTEARG,
 [
   AC_CHECK_FUNCS(isascii mbrtowc)
-  AC_CHECK_HEADERS(limits.h stdlib.h string.h wchar.h)
+  AC_CHECK_HEADERS(limits.h stdlib.h string.h wchar.h wctype.h)
   AC_HEADER_STDC
   AC_C_BACKSLASH_A
   AM_C_PROTOTYPES