Extending HashMap and adding JavaBean properties confuses Groovy

Let's say you have a class like so:

public class StupidClass extends HashMap
{
String myProperty = "Hello";

public String getMyProperty()

}

If you write, in a groovy script:

obj = new StupidClass();
return obj.myProperty;

It doesn't work - it seems to look in the map for "myProperty", and if it doesn't find it, accepts failure. Instead, I have to write:

obj = new StupidClass();
return obj.getMyProperty();

What I think would be appropriate is for Groovy to check for a javabean property first, and then for the Map key.