sqlalchemy custom string without any special symbol and spaces

I try to create SQLalchemy class except existing string type. My type should raise errors if there are special symbols or spaces in value. There is my implementation.

class StringWithoutSpecialSymbols(TypeDecorator):

impl = sa.String

def process_bind_param(self, value, dialect):
    if value:
        if re.match(r"^[a-z0-9A-Zа-яА-ЯёЁ]+$", value):
            return value
        raise ValueError("Object shouldn`t contain any special symbols or spaces")
    return value

def process_result_value(self, value, dialect):
    return value

def copy(self, **kw):
    return StringWithoutSpecialSymbols(self.impl.length)

But there is an error

/lib/python3.9/site-packages/flask_admin/contrib/sqla/view.py", line 719, in scaffold_filters
for f in flt:

TypeError: 'NoneType' object is not iterable