Trouble converting Household into Individual

Hi all,

I've never had this issue and cannot tell what is causing it. I am attempting to convert a Household record into an Individual record, and mapping the numerous existing affiliates accordingly. I receive the following Invalid Entry notice: The Converted household may only be merged with the A1 or A2 affiliate. 

As far as I can tell, I am correctly selecting the existing A1 to merge into the household account. Can anyone tell me what I am doing wrong?