I agree with Critter#2. If no allocations are made to the account for the plan year ending in or with the employee's taxable year, the instructions for Form W-2 indicate that the employer is not to mark box 13 Retirement plan. Allocations would include forfeitures by other participants, so it's not just employee and employer contributions. Since the IRS goes by box 13 Retirement plan, one would need to make sure the employer is marking or not marking box 13 Retirement plan as is appropriate for the circumstances.
