How to resolve the GroupMembershipController-error in my Rules Component?

I created a variation of the Rules Component in comment nr 5 of issue #2603136: Rules integration. Here is how my Rules Component looks like:

{ "rules_add_role_to_selected_group_members" : {
    "LABEL" : "Add role to selected group members",
    "PLUGIN" : "rule",
    "OWNER" : "rules",
    "REQUIRES" : [ "rules" ],
    "ACCESS_EXPOSED" : "1",
    "USES VARIABLES" : {
      "parm_group_mbrship" : { "label" : "Parm Group Membership", "type" : "group_membership" },
      "parm_group_role" : { "label" : "Parm Group Role machine name", "type" : "group_role" }
    },
    "IF" : [
      { "NOT list_contains" : {
          "list" : [ "parm-group-mbrship:roles" ],
          "item" : [ "parm-group-role" ]
        }
      }
    ],
    "DO" : [
      { "drupal_message" : { "message" : "Role data passed as parm:\u003Cbr \/\u003E\r\n\u003Cbr \/\u003E\r\nparm-group-role:rid = [parm-group-role:rid]\u003Cbr \/\u003E\r\nparm-group-role:group-type=[parm-group-role:group-type]\u003Cbr \/\u003E\r\nparm-group-role:label=[parm-group-role:label]\u003Cbr \/\u003E\r\n\u003Cbr \/\u003E" } },
      { "entity_fetch" : {
          "USING" : { "type" : "group_membership", "id" : [ "parm-group-mbrship:mid" ] },
          "PROVIDE" : { "entity_fetched" : { "membership_fetched" : "Fetched Membership" } }
        }
      },
      { "drupal_message" : { "message" : "Fetched membership data:\u003Cbr \/\u003E\r\n\u003Cbr \/\u003E\r\nmembership-fetched:mid = [membership-fetched:mid]\u003Cbr \/\u003E\r\nmembership-fetched:group=[membership-fetched:group]\u003Cbr \/\u003E\r\nmembership-fetched:roles=[membership-fetched:roles]\u003Cbr \/\u003E\r\n\u003Cbr \/\u003E" } },
      { "list_add" : {
          "list" : [ "membership-fetched:roles" ],
          "item" : [ "parm-group-role" ],
          "unique" : "1"
        }
      },
      { "entity_save" : { "data" : [ "membership-fetched" ], "immediate" : "1" } }
    ]
  }
}

To QA-test my Rules Component, I execute it from within the Rule UI, using its "execute" link to the right of the Rules Component, and I enter these arguments for it:

• For 'Parm Group Membership' I use 'Group membership identifier' = 7 (entered after switch to direct input mode)
• For 'Parm Group Role machine name' I use 'Group role identifier' = instructor

The "Display a message" actions are to facilitate debugging for now. Here is how a sample of them (when executing the Rules Component) looks like:

Role data passed as parm:

parm-group-role:rid = 3
parm-group-role:group-type=Class
parm-group-role:label=Instructor

Fetched membership data:

membership-fetched:mid = 7
membership-fetched:group=Drupal for Sitebuilders
membership-fetched:roles=Class: Student

However I run into an error which looks like so:

Recoverable fatal error: Object of class GroupRole could not be converted to string in GroupMembershipController->save() (line 141 of .../sites/all/modules/group/classes/group_membership.controller.inc).

To take this a step further, I also tried to execute this Rules Component using the "role id" of "3" (for the same "instructor" role), specified as parm for "parm_group_role". However in that case my "Display a message"-actions don't display anything at all. And instead I get these errors that show up if I enable Rules debugging:

0 ms Executing action rule: Add role to selected group members.
33.275 ms Unable to get a data value. Error: Invalid data value given. Be sure it matches the required data type and format. Value at group_role(): 3.
33.795 ms Unable to evaluate action component_rules_add_role_to_selected_group_members.

My questions:

1. How to resolve that GroupMembershipController-error?
2. What should I specify as the value for the "Group role identifier": instructor, 3, or yet something else?
3. How can I get my Rules Component to do what I want it to do, ie add a role for the Group Membership specified as a Parameter for my Rules Component?