diff --git a/src/standard.c b/src/standard.c
index 7a6ca11..37b3c43 100644
--- a/src/standard.c
+++ b/src/standard.c
@@ -731,7 +731,7 @@
 			goto out;
 		}
 
-		if (isdigit(*port1)) {	/* single port or range */
+		if (isdigit((int)(unsigned char)*port1)) {	/* single port or range */
 			port2 = strchr(port1, '-');
 			if (port2)
 				*port2++ = '\0';