diff --git a/include/mbedtls/check_config.h b/include/mbedtls/check_config.h
index fa7110f..301b283 100644
--- a/include/mbedtls/check_config.h
+++ b/include/mbedtls/check_config.h
@@ -30,6 +30,10 @@
 #ifndef MBEDTLS_CHECK_CONFIG_H
 #define MBEDTLS_CHECK_CONFIG_H
 
+#if defined(MBEDTLS_CONFIG_FILE) && defined(MBEDTLS_USER_CONFIG_FILE)
+#error "MBEDTLS_USER_CONFIG_FILE cannot be included if MBEDTLS_CONFIG_FILE is defined."
+#endif
+
 /*
  * We assume CHAR_BIT is 8 in many places. In practice, this is true on our
  * target platforms, so not an issue, but let's just be extra sure.