The current implementation of UUID validator at https://github.com/wtforms/wtforms/blob/master/src/wtforms/validators.py#L520 assumes that field.data is of type str. However, if field.data is already an instance of uuid.UUID the validator fails with the following exception: AttributeError: 'UUID' object has no attribute 'replace'.

I've found this issue working with PostgreSQL, SQLAlchemy and using Flask-Admin with a field declared as:

    from sqlalchemy.dialects.postgresql import UUID
    import sqlalchemy as sa

    # ... More code here
    uuid = db.Column(
        UUID(as_uuid=True), nullable=False, default=uuid4,
        server_default=sa.text('uuid_generate_v4()'), index=True)

Note the use of as_uuid=True as said at pallets-eco/flask-admin#1444.

My proposal is to change this line uuid.UUID(field.data) by uuid.UUID(str(field.data)), this way it should work in both cases.

Meanwhile, I've done the following workaround implementing a new UUID validator overriding the __call__() method as follows:

    import uuid

    from wtforms.validators import UUID


    class SecureUUID(UUID):
        def __call__(self, form, field):
            if isinstance(field.data, uuid.UUID):
                field.data = str(field.data)
            super().__call__(form, field)