Calendar NOT losing focus WPF 4.0

I have a WPF 4.0 Calendar control (Selection Mode: MultipleRange) and a button in a grid.

After selecting dates in the calendar, when I click on the button, the calendar is not losing its focus.  i.e., It leads me to click twice on the button to fire the button_click event.

Is that a known bug? I binged my level best but not finding any answer. I will appreciate if someone could solve it.

Thank you.