Error #1034: Type Coercion failed: cannot convert

programming flash
1

I have done a test code to create plattform to move down and then go back to the top.
It works, but when i copy the code and paste it for the game then I get this error.

TypeError: Error #1034: Type Coercion failed: cannot convert flash.display::MovieClip@3633941 to plattform.

I have localized who cause the problem but I don’t know how to fix it.
Here is the problem

    var tempPlatt:plattform = plattform(this.getChildAt(j));

The rest of the code

//Create plattform
for(var i:int=0;i<6;i++) {
    var myPlatt:plattform = new plattform();

    myPlatt.x = Math.random() *330+10;
    
    myPlatt.y = -650+i*100;
    myPlatt.name = "minPlatt" +i;
    addChild(myPlatt);
}



this.addEventListener(Event.ENTER_FRAME, onEnterFrameHandler);

function onEnterFrameHandler(evt:Event):void {
    for(var j:int=0;j<this.numChildren;j++) {
    var tempPlatt:plattform = plattform(this.getChildAt(j));

//Change Speed
    tempPlatt.y +=1.5;
    
        if(tempPlatt.y >550){
            tempPlatt.x = Math.random() *330+10;
            tempPlatt.y =-50;
            trace(tempPlatt.x);
            trace(tempPlatt.name);
        }
    }
}