Please help me if you can!

I have a formula to display two dates and some text if the dates are not null. But it is not working. The formula is this:

If isNull({reportview_TimeToTransfer.ExStartDate})
and
isNull({reportview_TimeToTransfer.ExEndDate}) Then
' '
Else
CStr({reportview_TimeToTransfer.ExStartDate}) & ' - '
& CStr({reportview_TimeToTransfer.ExEndDate}) & ' (Extension)'

If it worked correctly, it should show:

01/01/2002 - 12/01/2002 (Extension)

or if the dates are null, it should be blank. But if the dates are null, it is showing:

- (Extension)

Any ideas anyone?!