Seems to be a bug. It was reported above. It happens for mixed race or Hybrid race characters.
Until it's fixed, you can fix the problem by going into your save file with notepad, finding that bred character and changing their race_display from mixed or whatever it is to a specific race.
Viewing post in Aric's Expansion Rebred
I had the same issue and can confirm that this fixes the issue (at least that it makes you able to interact with them). One thing to note is that for me it seemed that I had to select the race that was listed as 'primary' in the field above/before the 'race_display' field. Trying it the other way (i.e. using the secondary race) had the same error occurring for me.