Examples of abnormal behavior in Java

Can anyone explain why the following if statement evaluates to false?

public void addShapeToWhiteboard(PolyLine shape)
{
   Window.alert("2");
   if(shape instanceof PolyLine)
   {
      Window.alert("3");
      this.whiteboard.add((PolyLine)shape);
      Window.alert("3.5");
   }    
   this.whiteboard.draw();
   Window.alert("4");
}

It needs a "polyline" object, but instanceof returns false because I receive an alarm of "2", then an alarm of "4", and there is no clue, or even possible

Solution

Maybe the shape is empty? In this case, instanceof returns false