diff --git a/src/category.c b/src/category.c
index b7eadd0e771..1cae5a6e756 100644
--- a/src/category.c
+++ b/src/category.c
@@ -67,7 +67,7 @@ those categories.  */)
   val = MAKE_CATEGORY_SET;
 
   if (STRING_MULTIBYTE (categories))
-    error ("Multibyte string in make-category-set");
+    error ("Multibyte string in `make-category-set'");
 
   len = SCHARS (categories);
   while (--len >= 0)