public function prepareValueAndOperator($value, $operator, $useDefault = false)
{
    if ($useDefault) {
        return [$operator, '='];
    } elseif ($this->invalidOperatorAndValue($operator, $value)) {
        throw new InvalidArgumentException('Illegal operator and value combination.');
    }

    return [$value, $operator];
}

In this example you cannot execute a WHERE condition on a NULL value:

->where('price', '<=', $fPrice)