diff --git a/lib/str-utils.h b/lib/str-utils.h
index f8ea17753..317034979 100644
--- a/lib/str-utils.h
+++ b/lib/str-utils.h
@@ -169,7 +169,7 @@ ch_isalpha(gchar c)
 static inline gboolean
 ch_isascii(gchar c)
 {
-  return c >= 0 && c <= 127;
+  return c >= 0 && (guchar ) c <= 127;
 }
 
 static inline gchar