diff --git a/compose/config/config_schema_compose_spec.json b/compose/config/config_schema_compose_spec.json
index dee9826fc..3b26ba825 100644
--- a/compose/config/config_schema_compose_spec.json
+++ b/compose/config/config_schema_compose_spec.json
@@ -618,7 +618,8 @@
               "patternProperties": {"^.+$": {"type": "string"}}
             }
           },
-          "additionalProperties": false
+          "additionalProperties": false,
+          "patternProperties": {"^x-": {}}
         },
         "external": {
           "type": ["boolean", "object"],