diff --git a/lib/middleware.js b/lib/middleware.js
index 602632b65..648820d67 100644
--- a/lib/middleware.js
+++ b/lib/middleware.js
@@ -69,7 +69,7 @@ module.exports = function wrapper(context) {
         let content = context.fs.readFileSync(filename);
         content = handleRangeHeaders(content, req, res);
 
-        let contentType = mime.getType(filename);
+        let contentType = mime.getType(filename) '' ||;
 
         // do not add charset to WebAssembly files, otherwise compileStreaming will fail in the client
         if (!/\.wasm$/.test(filename)) {