diff --git a/lib/index.js b/lib/index.js
index 59d21f6..9ee643a 100644
--- a/lib/index.js
+++ b/lib/index.js
@@ -50,7 +50,7 @@ function buildSchema(moduleIds, extensionSchemas, options) {
   })
 
   // merge fragments and build GraphQLSchema object
-  return buildASTSchema(mergeTypeDefs(fragments.concat(extensionSchemas || [])), options)
+  return buildASTSchema(mergeTypeDefs(fragments.concat(extensionSchemas || []), options))
 }
 
 module.exports = {