diff --git a/lib/propobj.js b/lib/propobj.js
index 6a78020..647cb19 100644
--- a/lib/propobj.js
+++ b/lib/propobj.js
@@ -78,14 +78,17 @@ exports.setup = function( dest, properties, methods )
 /**
  * Set up destination object by copying over properties and methods
  *
- * @param  {Object}   dest        destination object
- * @param  {Object}   properties  properties to copy
- * @param  {Object=}  methods     methods to copy
+ * @param  {Object}   dest           destination object
+ * @param  {Object}   properties     properties to copy
+ * @param  {Object=}  methods        methods to copy
+ * @param  {boolean}  unless_exists  do not set if method already exists in dest
  *
  * @return  {undefined}
  */
 function doSetup( dest, properties, methods, unless_exists )
 {
+    unless_exists = !!unless_exists;
+
     var hasOwn = Array.prototype.hasOwnProperty;
 
     // copy over the methods