diff --git a/lib/dns/rdata/in_1/svcb_64.c b/lib/dns/rdata/in_1/svcb_64.c
index 12e8caae52..a9d65f98da 100644
--- a/lib/dns/rdata/in_1/svcb_64.c
+++ b/lib/dns/rdata/in_1/svcb_64.c
@@ -256,7 +256,7 @@ svc_fromtext(isc_textregion_t *region, isc_buffer_t *target) {
 			RETERR(alpn_fromtxt(region, target));
 			break;
 		case sbpr_port:
-			if (!isdigit(*region->base)) {
+			if (!isdigit((unsigned char)*region->base)) {
 				return (DNS_R_SYNTAX);
 			}
 			ul = strtoul(region->base, &e, 10);