diff --git a/src/core.js b/src/core.js
index 1184ee02..ace2fda0 100644
--- a/src/core.js
+++ b/src/core.js
@@ -47,7 +47,7 @@ module.exports = function core(defaultLibraryName) {
         let modulePathTpl;
         let styleRoot;
         let mixin = false;
-        let ext = options.ext || '.css';
+        const ext = options.ext || '.css';
 
         if (root) {
           _root = `/${root}`;