When importing XPO file, compare tool didn't response for a class

Hi. Experts:

I and using AX 2009.

I had a weird question, When I import a XPO file.

There is a class when I select to compare,

No window is display to select source and destination.
It just nothing happen. and only this class has the situation.

Have anyone has any idea what's wrong with this class?

Thanks a lot!

Quatrani