SQLAlchemy Numeric deserialized as string with auto_field

[Anti-Distinctlyminty]

I have an SQLAlchemy mode with a Numeric column as in the following example

db = flask_sqlalchemy.SQLAlchemy()
ma = flask_marshmallow.Marshmallow()

class Project(db.Model):
    budget = db.Column(db.Numeric)

class ProjectSchema(ma.SQLAlchemyAutoSchema):
    class Meta:
        model = Project
    budget = ma.auto_field()

Deserialized values for the budget field appear as strings. However, if I remove the auto_field and use Number instead, all is well. Is there a reason why db.Numeric doesn't map to ma.Number?