Assign Review Pool JSON API

Hello,

I'm trying to use the JSON API to assign a review pool to a role in a review

here is my call:

{"command": "ReviewService.assignReviewPool",

"args" : {

    "reviewId" : "123"

    "assignment" : {

        "poolGuid" : "123456abcdef"

        "role" : "REVIEWER"

    }

}}

And this is the response it returns:

{"errors": [{"code": "ReviewPoolNotFound", "message": "No review pool assignments with Group guid "123456abcdef" and Role REVIEWER were found in Review 123, or the given user does not have the right group membership to get that assignment"}]}

I've double checked the group guid and the role, both seem to be correct. I'm also pretty sure I should have the correct permissions to execute this, I am able to assign the group to the role from within the GUI

Is there something wrong with how I am making this call? 

Thanks!