diff --git a/src/apps/altcp_tls/altcp_tls_mbedtls.c b/src/apps/altcp_tls/altcp_tls_mbedtls.c
index 9e619126..6e6189ee 100644
--- a/src/apps/altcp_tls/altcp_tls_mbedtls.c
+++ b/src/apps/altcp_tls/altcp_tls_mbedtls.c
@@ -422,7 +422,7 @@ altcp_mbedtls_handle_rx_appldata(struct altcp_pcb *conn, altcp_mbedtls_state_t *
     } else {
       err_t err;
       if (ret) {
-        LWIP_ASSERT("bogus receive length", ret <= PBUF_POOL_BUFSIZE);
+        LWIP_ASSERT("bogus receive length", (size_t)ret <= PBUF_POOL_BUFSIZE);
         /* trim pool pbuf to actually decoded length */
         pbuf_realloc(buf, (u16_t)ret);