diff --git a/parse.c b/parse.c
index f52139f..239e371 100644
--- a/parse.c
+++ b/parse.c
@@ -199,7 +199,7 @@
 		if (!isdigit((int) *p) &&
 		    (((*p != '+') && (*p != '-')) || digit_seen))
 			break;
-		digit_seen |= isdigit(*p);
+		digit_seen |= isdigit((int) *p);
 		p++;
 	}