diff --git a/lib/less/parser.js b/lib/less/parser.js
index 1fcd4f7..3210848 100644
--- a/lib/less/parser.js
+++ b/lib/less/parser.js
@@ -178,7 +178,7 @@ less.Parser = function Parser(env) {
     // the individual nodes in the tree.
     this.optimization = ('optimization' in this.env) ? this.env.optimization : 1;
 
-    this.env.filename = this.env.filename || __filename;
+    this.env.filename = this.env.filename || null;
 
     //
     // The Parser