Tag property of a User Control

I have to modify an existing user control in such a way that I don't break binary compatibility since the control is used all over the place and there are 3000 distributed end users of the application. I wanted to be able to set a value into the controls Tag then evaluate it at runtime. However the property Let never fires. If I create my own Tag property (eg MyTag) its fine. Is there something restriction with the tag in a user control? I am using VB5 SP3 (cant upgrade to 6 yet)