validate combo on button click in wpf mvvm

I am creating user control , in which I have placed combo, binded with Enumerations. Binding this enum list has no error.

I try to validate selectedvalue of this combo in button click. But this throw's error while page loads as ,
Value " could not be converted.

If I select 1st option (none) from the list , this error message disapears.
I think, selectedvalue property takes empty string in some where, I can't traceout this.

My coding is as below