Why does TurboTax ask for RMD for a pension?

I received a 1099-R for a state pension distribution.  After putting this form into TurboTax, it asked whether any of this distribution was for an RMD.  Because I am older than 70.5 years, TT says I have to take an RMD for this account.  I have no idea what the RMD is for this account.  Because it is a pension, rather than an IRA, I don't think I have a previous year balance on which to compute a RMD value.  I just get a monthly payment for life.  What do I do?