diff --git a/src/mbyte.c b/src/mbyte.c
index 2b7f999..a01a051 100644
--- a/src/mbyte.c
+++ b/src/mbyte.c
@@ -4226,7 +4226,7 @@
     convert_setup(&vimconv, NULL, NULL);
 }
 
-#if defined(FEAT_GUI_GTK) || defined(PROTO)
+#if defined(FEAT_GUI_GTK) || defined(FEAT_SPELL) || defined(PROTO)
 /*
  * Return TRUE if string "s" is a valid utf-8 string.
  * When "end" is NULL stop at the first NUL.