diff --git a/src/if-options.c b/src/if-options.c
index 06ef1cb8..1b256fa9 100644
--- a/src/if-options.c
+++ b/src/if-options.c
@@ -1084,7 +1084,7 @@ parse_option(struct dhcpcd_ctx *ctx, const char *ifname, struct if_options *ifo,
 		}
 
 		arg = p + 1;
-		s = sizeof(ifo->vendor) - 1 - ifo->vendor[0] - 2;
+		s = (ssize_t)sizeof(ifo->vendor) - 1 - ifo->vendor[0] - 2;
 		if (inet_aton(arg, &addr) == 1) {
 			if (s < 6) {
 				s = -1;