diff --git a/sys/net/gnrc/transport_layer/tcp/gnrc_tcp_option.c b/sys/net/gnrc/transport_layer/tcp/gnrc_tcp_option.c
index 14b4ae25d7..340b92abb9 100644
--- a/sys/net/gnrc/transport_layer/tcp/gnrc_tcp_option.c
+++ b/sys/net/gnrc/transport_layer/tcp/gnrc_tcp_option.c
@@ -63,6 +63,7 @@ int _option_parse(gnrc_tcp_tcb_t *tcb, tcp_hdr_t *hdr)
             default:
                 DEBUG("gnrc_tcp_option.c : _option_parse() : Unknown option found.\
                       KIND=%"PRIu8", LENGTH=%"PRIu8"\n", option->kind, option->length);
+                return -1;
         }
 
         if (option->length > opt_left) {