Roth IRA

At the age of 64 I made a Roth IRA withdrawal. The 1099-R I received from Fidelity has the taxable income, box 2a, as 0, Box 5 states Roth contribution is equal with distribution, and box 7 is filled with "B" which indicates a Roth distribution.

The IRS wants to tax the distribution saying they can't determine if the distribution is a pension or an annuity. 

Having sent a couple explanatory responses I am now going to call them.

Any thoughts on why the IRS is not seeing this as a nontaxable Roth distribution?