Redmine

I'm trying to integrate our LMS with redmine for our project course, and am using the REST API. user creation and project creation works nicely, but when trying to add a user to a project the following error occurs

Started POST "/redmine/projects/project-2/memberships.xml?key=xxxxxxxxxxxxxxxxxxxxxxx" for 127.0.0.1 at 2014-11-02 11:55:01 +0530
Processing by MembersController#create as XML
Parameters: {"membership"=>{"userid"=>"8", "role_ids"=>{"role_id"=>"6"}}, "key"=>"xxxxxxxxxxxxxxxxxxxxxxx", "project_id"=>"project-2"}
Current user: root (id=1)
Completed 500 Internal Server Error in 138.0ms

NoMethodError (undefined method `to_i' for ["role_id", "6"]:Array):
app/models/member.rb:44:in `each'
app/models/member.rb:44:in `collect'
app/models/member.rb:44:in `role_ids='
app/controllers/members_controller.rb:59:in `new'
app/controllers/members_controller.rb:59:in `create'

any ideas whats going on?