Code B in box 7 with $0 in box 1 and an amount in box 5 reports a distribution of unrecoverable basis in designated Roth contributions. However, such a form makes no sense unless there was another distribution for a direct rollover from a designated Roth account to a Roth IRA, reported on a Form 1099-R with code H in box 7, or a direct rollover from a designated Roth account to a designated Roth account in another employer's plan, reported with codes B and G in box 7. The code-G Form 1099-R reports a distribution from the traditional account in your 401(k), not from a designated Roth account in the 401(k). (The only reason I can think of where such a code-B Form 1099-R would be issued without any other distribution from the designated Roth account is if the entire investment in the designated Roth account became worthless, but that's implausible unless the entire investment consisted of company stock which somehow became worthless.)
