Why CDaoRecordset::IsFieldNull is not const? Is bad practice to cast it to const?

I need constantly query DAO fields to check if they are NULL. Why CDaoRecordset::IsFieldNull is not const? Is bad practice to cast it to const in order to enable to declare my member class as const, since (I suppose) nothing really changes when I call IsFieldNull(). In short, is it a bad coding practice? Example:

bool CDaoQuery::MyFieldIsValid() const
{
    CDaoQuery& rqry = const_cast<CDaoQuery&>(*this);        // override the const

    return rqry.IsFieldNull(&rqry.m_Field) == FALSE;
}

or...

bool CDaoQuery::MyFieldIsValid()
{
    return IsFieldNull(&m_Field) == FALSE;
}