I am trying to deploy new functionality to our production instance with a number of custome objects. One fo the objects has 2 master-detail fields as well as a Lookup field. When attempting to deploy using change sets I get the following error message related to the Lookup field:

Cannot set sharingModel to ControlledByParent on a CustomObject without a MasterDetail relationship field

I cannot change the lookup field to a MasterDetail type and I cannot figure out how to change The sharingModel for that object, under the sharing settings it says "You cannot create sharing rules for this item." under the Sharing Rules section.