Is this vs2005 beta 2 bug or my fault?

when i try to implement the interface System.Collections.Generic.IList.
i met this error:

'ClassLibrary1.CustomList' does not implement interface member 'System.Collections.IEnumerable.GetEnumerator()'. 'ClassLibrary1.CustomList.GetEnumerator()' is either static, not public, or has the wrong return type.

and my code just like this:

public IEnumerator GetEnumerator()
{
return new CustomEnumerator(_field1);
}

what's wrong happened?