diff --git a/lib/less/parser.js b/lib/less/parser.js
index b7cbde4..14b7fba 100644
--- a/lib/less/parser.js
+++ b/lib/less/parser.js
@@ -170,7 +170,7 @@ less.Parser = function Parser(env) {
         }
     }
 
-    this.env = env || {};
+    this.env = env = env || {};
 
     // The optimization level dictates the thoroughness of the parser,
     // the lower the number, the less nodes it will create in the tree.
@@ -178,6 +178,8 @@ 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;
+
     //
     // The Parser
     //