The original saved FAST! data sheet had 1099 fields that were removed and may be causing the exception, create a new FAST! data sheet:
  1. In FAST! click New AP invoice button to create a new datasheet without any 1099 fields
  2. Save and Close
OR

Change the 1099 distribution column in the FAST Datasheet view to be <None> on the blue default line at the top before starting to enter invoices.

(This option will appear if we are selecting the "1099 Distribution" field that appears in blue under the Invoice section of available fields on the design tab)