Web designer window not opening if inherited from another web form class

Hi everybody,
I created a web form (WFrmEnt) and then used the MustInherit clause to make the webform class to an abstract base class. I then inherited WFrmEnt class in another web form (ItemEnt) which has additional controls to those present in WFrmEnt. ItemEnt runs perfectly fine, but its designer window does not open and shows the error message:
"The file failed to load in the Web designer. Please correct the following error, then load it again. Type Abstract." What could be the problem?

The designer doesn't know what type of class it is anymore.

Why does it happen only with this class and not with other classes?
What is the solution to this problem?