diff --git a/acinclude.m4 b/acinclude.m4
index bea7f1d7f..120dd82ac 100644
--- a/acinclude.m4
+++ b/acinclude.m4
@@ -53,7 +53,7 @@ AC_DEFUN([GUILE_HEADER_LIBC_WITH_UNISTD],
   [
     AC_CHECK_HEADERS(libc.h unistd.h)
     AC_CACHE_CHECK(
-      "whether libc.h and unistd.h can be included together",
+      [whether libc.h and unistd.h can be included together],
       guile_cv_header_libc_with_unistd,
       [
         if test "$ac_cv_header_libc_h" = "no"; then