That's a bug in the Online software that's just been discovered in the last few days as the Brokerages just released millions of 1099-DIV forms over the weekend.. (not a problem with the Desktop software).
It may take a few weeks to get fixed....check on Fridays, since software updates usually occur only on Wed or Thurs evenings.
See discussion here where the software BUG was confirmed:
We found a workaround:
go back and
1) Edit the 1099-DIV in question
2) On the page that shows boxes 1a, 1b and 2a....just click on the box to show all the other boxes ( you apparently don't need ) "My form has info in other boxes"
3) Scroll to the bottom of the page, and hit "Continue"
4) Next page make sure you have the "A portion of these dividends is U.S. Government interest" and hit "Continue"
5) Enter the US govt interest in the box provided and continue out to the 1099-DIV table.
should be OK now.......don't uncheck that box in step #2 or it will go away again.