diff --git a/src/wayland-server.c b/src/wayland-server.c
index eb1e5000..eab2ee45 100644
--- a/src/wayland-server.c
+++ b/src/wayland-server.c
@@ -353,7 +353,7 @@ wl_client_connection_data(int fd, uint32_t mask, void *data)
 		}
 	}
 
-	while ((size_t) len >= sizeof p) {
+	while (len >= 0 && (size_t) len >= sizeof p) {
 		wl_connection_copy(connection, p, sizeof p);
 		opcode = p[1] & 0xffff;
 		size = p[1] >> 16;