Can't delete a contact from a group

I took the following steps:
* I am on a group page: e.g. https://go.vumi.org/contacts/group/5844c59dd26045b2955ee284413e3f3e/
* I check the box next to a contact row
* I click the delete btn in the header
* I get an error screen (attached)

Note: I don't get this error when deleting a contact from the contacts screen (i.e. not from a group)

![screen shot 2013-08-26 at 1 21 40 pm](https://f.cloud.github.com/assets/1521387/1025361/afa3e178-0e41-11e3-8c7f-df18b2d7bbfb.png)

![screen shot 2013-08-26 at 1 19 36 pm](https://f.cloud.github.com/assets/1521387/1025362/b6760c9c-0e41-11e3-889b-68535b2a80b7.png)