Clear relationship by sending null data with PUT doesn't work

[Zooip]

In JSON-API specs, it is written that the following request must clear my relationship

PATCH /articles/1/relationships/author HTTP/1.1
Content-Type: application/vnd.api+json
Accept: application/vnd.api+json

{
  "data": null
}

When I try, i get a 500 error :

"exception": "undefined method `[]' for nil:NilClass",
"backtrace": [
          "**/jsonapi-resources-0.7.0/lib/jsonapi/request.rb:589:in `parse_update_relationship_operation'",
          ....
  ]

On the other way, when I send an update request with a non-existing id, server responds with 204 No Content and the relationship is set to nil. I would have expected a 422 error but the JSON-API doesn't specifies anything on this.

Did I get something wrong ?

[Zooip]

After some tests, error seams to appear only with my polymorphic associations.
https://github.com/cerebris/jsonapi-resources/blob/master/lib/jsonapi/request.rb#L587

It doesn't verify presence of verified_params[:to_one].values[0]