Error when accepting/rejecting an invitation to a subwiki created by a non Admin user

Steps to reproduce:
1. Create a new subwiki with a non Admin user
2. Check this option from Configuration: Any global user can request to join but an admin must accept and Only global users are available in the wiki
3. Login with a different user
4. Go to Wiki Index an select Join Request for that subwiki just created
5. Logout
6. Login with the owner of the subwiki
7. Go to Administer Wiki
8. Click on Users
9. Go to candidates and Accept/Reject the invitation

You will receive an error when you try to accept/reject the invitation and the invitation still remains in pending. The same thing happen when you are logged in with an Admin user on a subwiki created by a non Admin user.
See the picture attached.