This was found in svn r13117 and has been reproduced in svn r13238. 

Bug details: Using the automatic decimal places option raises an error in the calculator unless a decimal point is put in!

To reproduce: 
1. Edit -> Preferences -> General, set automatic decimal places to ON.
2. Start the Financial Calculator from the Tools menu.
3. Click 'Clear' for Payment periods and Future value.
4. Enter the number of payments as '12'
5. Click Calculate.
6. Payment periods is reset to 0.00
7. An error message dialogue appears because the number of payments cannot be zero. (!)

The calculator works fine without automatic decimal places or if the number is
entered as '12.' when automatic decimal places is enabled, but that's not what
a user would expect from enabling automatic decimal places!