pk appear in jsonschema

[Mograbi]

after creating an object of JsonModel I want to have the jsonschema for this model
when running schema_json() I get a "pk" as part of the schema which shouldn't be

{
   "title":"Callback",
   "type":"object",
   "properties":{
      "pk":{
         "title":"Pk",
         "type":"string"
      },
      ....