Nesting generic fields in Marshmallow

I am trying the marshmallow ORM library from here. I have a general reporting schema with this form:

    class ReportingQueryResultSchema(ma.Schema):
         start_date = fields.Str(required=True)
         end_date = fields.Str(required=True)
         data = fields.Nested(
             ReportingDataSchema,
             required=True,
             many=True
        )

ReportingDataSchema is a generic field. It can be different types of schema. My first instinct here is inheritance. So I try something like this:

    class ReportingDataSchema(ma.Schema):
        pass

    class FooSchema(ReportingDataSchema):
        value = fields.Str()

    class BarSchema(ReportingDataSchema):
        thing = fields.Str()

marshmallow fails to serialize the nested fields, giving me the following:

    {
         "data": [
              {}
         ],
        "end_date": "2017-06-05",
        "start_date": "2017-06-01"
    }

But if I declare ReportingQueryResultSchema as:

    class ReportingQueryResultSchema(ma.Schema):
        start_date = fields.Str(required=True)
        end_date = fields.Str(required=True)
        data = fields.Nested(
             FooSchema,   # replaced here
             required=True,
             many=True
        )

It is then able to detect the nested schema and properly populate the result.

    {
         "data": [
              {"key": "val"}
         ],
        "end_date": "2017-06-05",
        "start_date": "2017-06-01"
    }

What is the correct way to do generic nested schema in marshmallow?