diff --git a/lib/Support/regcomp.c b/lib/Support/regcomp.c
index cd018d5dc5b..46c91a9c497 100644
--- a/lib/Support/regcomp.c
+++ b/lib/Support/regcomp.c
@@ -780,7 +780,7 @@ p_b_cclass(struct parse *p, cset *cs)
 	const char *u;
 	char c;
 
-	while (MORE() && isalpha(PEEK()))
+	while (MORE() && isalpha((uch)PEEK()))
 		NEXT();
 	len = p->next - sp;
 	for (cp = cclasses; cp->name != NULL; cp++)