diff --git a/lib/src/parser.c b/lib/src/parser.c
index f3eaf193..4d64f373 100644
--- a/lib/src/parser.c
+++ b/lib/src/parser.c
@@ -397,10 +397,12 @@ static unsigned ts_parser__external_scanner_serialize(
       self->lexer.debug_buffer
     );
   } else {
-    return self->language->external_scanner.serialize(
+    uint32_t length = self->language->external_scanner.serialize(
       self->external_scanner_payload,
       self->lexer.debug_buffer
     );
+    assert(length <= TREE_SITTER_SERIALIZATION_BUFFER_SIZE);
+    return length;
   }
 }