Is this normal an object property is not allowed in an ActiveX control?

This is my definition for the property:

Public Property Get ParentObject() As Object
ParentObject = loParentObject
End Property

Public Property Let ParentObject(ByRef loValue As Object)
Set loParentObject = loValue
PropertyChanged "ParentObject"
End Property

If I change it for String, it works. Any idea on how I can make it work with an object?