After upgrade from B1 to B2 my app stopped working, throwing ConversionException on nullable DateTime field, value of which is null. There is part of DateTimeType.php which causes it - if (!$val) matches even the valid null value. public function convertToPHPValue($value, AbstractPlatform $platform) { $val = ($value !== null) ? \DateTime::createFromFormat($platform->getDateTimeFormatString(), $value) : null; if (!$val) { throw ConversionException::conversionFailed($value, $this->getName()); } return $val; }