How to close an excel window

In my web application, I'm clicking on an "Export" button. Opens an excel window.  I give the window focus and then try to cancel out of the excel window that popped up.

I get 2 different errors:

1.  There was an attempt to perform an action on a zero-based window

or

2. There was an attempt to perform an action at point (#,#) which is overlapped by another window.

Any advice, workarounds, suggestions to get this excel window to close or cancel...?  Thanks!