diff --git a/include/mbedtls/check_config.h b/include/mbedtls/check_config.h
index 34f1a3b..00fb9b3 100644
--- a/include/mbedtls/check_config.h
+++ b/include/mbedtls/check_config.h
@@ -565,6 +565,12 @@
 #error "MBEDTLS_SSL_TLS_C defined, but no protocol version is active"
 #endif
 
+/* PROTO_TLS is not a documented option so far, but still check for conflicts
+ * involving it, in preparation for making it the documented option later */
+#if defined(MBEDTLS_SSL_PROTO_TLS) && defined(MBEDTLS_SSL_PROTO_NO_TLS)
+#error "MBEDTLS_SSL_PROTO_TLS and MBEDTLS_SSL_PROTO_NO_TLS both defined"
+#endif
+
 #if defined(MBEDTLS_SSL_TLS_C) && \
     ( defined(MBEDTLS_SSL_PROTO_NO_TLS) && !defined(MBEDTLS_SSL_PROTO_DTLS) )
 #error "MBEDTLS_SSL_TLS_C defined, but neither TLS or DTLS is active"