Default value for custom input object type

[tlinhart]

I have a custom input object type

class Filters(graphene.InputObjectType):                                        
    name = graphene.String()
    type = graphene.List(graphene.String)

and the query definition

class Query(graphene.ObjectType):                                               
    search = graphene.Field(
        SearchResult, query=graphene.String(required=True),
        filters=graphene.Argument(Filters, default_value=None)
    )

I want to be able to provide an empty default value for the filters argument. With the above definition, the argument is not set as optional and I'm getting this error message:

{
  "errors": [
    {
      "message": "resolve_search() missing 1 required positional argument: 'filters'",
      "locations": [
        {
          "line": 15,
          "column": 3
        }
      ],
      "path": [
        "search"
      ]
    }
  ],
  "data": {
    "search": null
  }
}

What is the correct way to provide an empty default value for the argument of custom object type? I'm at these package versions:

graphene==2.1.3 graphql-core==2.1 graphql-server-core==1.1.1