diff --git a/lib/ast.js b/lib/ast.js
index 42506cb2..61643aed 100644
--- a/lib/ast.js
+++ b/lib/ast.js
@@ -81,7 +81,9 @@ function DEFNODE(type, props, methods, base) {
     ctor.DEFMETHOD = function(name, method) {
         this.prototype[name] = method;
     };
-    exports["AST_" + type] = ctor;
+    if (typeof exports !== "undefined") {
+        exports["AST_" + type] = ctor;
+    }
     return ctor;
 };