I ran into the same problem today, but involving a UK entity with no Federal ID number. Strictly following the IRS recommendation would result in simply leaving Box 6 blank, but the Turbotax software will not allow that to happen. Most frustrating. However, I tried entering the number 99-0999999 as previously recommended on this thread and I am happy to report that it worked and the e-file went through. While I am extremely grateful to the person who suggested this work-around, I find it unacceptable that this problem has been around for a number of years without being fixed by Turbotax. So guys, hope you are reading these threads and sort this one out before next year rolls around.
