AS 3 functions optional value getting error

I have a gui class, The functions optional value getting error. If i am not passing the fillcolor and other optional values.

Error:

ReferenceError: Error #1069: Property fillColor not found on String and there is no default value.

AS 3.0

    var windowBase:Sprite = UIClip("Sprite");
    /* sliderClip = Gui开发者_StackOverflow.UIClip("MovieClip",{width:100, height:50, fillColor:0xFFFF0D, alpha:.7});
    */
    function UIClip (type:String, params:object = '') {
       var clip;
       trace("Hello")
       if (type == 'MovieClip') {
        clip = new MovieClip  ;
       } else {
        clip = new Sprite;
       }
       //trace(params + "params.fillColor " + params.fillColor)
       if (params is Object) {
        clip.graphics.beginFill ((params.fillColor != "") ? params.fillColor : 0xFFFFFF, params.alpha ? params.alpha : 1 );
        clip.graphics.lineStyle (params.lineThickness != "" ? params.lineThickness : 1, params.borderColor ? params.borderColor: 0x000000);
        clip.graphics.drawRoundRect (0,0, 
         (params.width != undefined ) ? params.width : 100, 
         (params.height != undefined) ? params.height : 100 ,
         params.eW ? params.eW : 0,
         params.eH ? params.eH : 0);
        clip.graphics.endFill ();
        //trace("Hello")
       }

       return clip;
      }

How can I solve this?

Condition "if (params is Object)" is true. String is object, too. I'm using null for optional params if they have no default value.

Try this: function UIClip (type:String, params:object = null) and test "if (params != null)"