It's more likely that the 1099-R is being treated as Non-Taxable because Box 2a is blank.
If Box 2b is marked "Taxable Amount Not Determined," then the amount in Box 1 of your 1099-R should be entered in Box 2a, unless you have information to indicate a different taxable amount.
If the box for "Taxable Amount Not Determined" is not checked, then the custodian of your husband's account has determined you RMD is not taxable.
My condolences on your loss.
