Daily Show's Jessica Williams???

I was just on the movie's Facebook page where they post pictures of the childrens' profiles.

Today they posted one for #247 Tanya Jackson. The actress playing her is The Daily Show's Jessica Williams. Why isn't she in the cast listing?

http://sphotos-a.xx.fbcdn.net/hphotos-ash3/1229998_159214810944436_933 465457_n.jpg