diff --git a/test/mocha/mangle-debug.js b/test/mocha/mangle-debug.js
index 6b1c8405..45e8b3cb 100644
--- a/test/mocha/mangle-debug.js
+++ b/test/mocha/mangle-debug.js
@@ -11,7 +11,7 @@ describe("mangle_properties 'debug' option ", function() {
             debug: true
         });
         
-        let stream = Uglify.OutputStream();
+        var stream = Uglify.OutputStream();
         ast.print(stream);
         var result = stream.toString();
         
@@ -33,7 +33,7 @@ describe("mangle_properties 'debug' option ", function() {
             debug: true
         });
         
-        let stream = Uglify.OutputStream();
+        var stream = Uglify.OutputStream();
         ast.print(stream);
         var result = stream.toString();