Note that here preverbal a- is not present. In the affirmative it is often used, but its true function remains a mystery. Thus:
I ten mamá. 'I have a mother'
I a-ten mamá. 'I have a mother'
BUT:
I ten mamá nu.
*I a-ten mamá nu. --> It is not clear whether this is or is not ungrammatical (i.e. it is never used).