diff --git a/src/if-options.c b/src/if-options.c
index 7b445f45..0b4d5deb 100644
--- a/src/if-options.c
+++ b/src/if-options.c
@@ -1126,6 +1126,10 @@ parse_option(struct dhcpcd_ctx *ctx, const char *ifname, struct if_options *ifo,
 
 		arg = p + 1;
 		s = (ssize_t)sizeof(ifo->vendor) - 1 - ifo->vendor[0] - 2;
+		if (s < 0) {
+			logerrx("vendor option list is full");
+			return -1;
+		}
 		if (inet_aton(arg, &addr) == 1) {
 			if (s < 6) {
 				s = -1;