Shiism is a vast doctrine that includes many subsects. Some of these subsects are moderate and have slight differences from the Sunni doctrine. Others are so extreme in such a way that they transgresses the basic principles of the Islamic belief insofar that the adherents of these extreme sects have stepped outside the fold of Islam.

Responding to this in more detail, Dr. Hatem Al-Haj, professor of fiqh said:

The validity of praying behind an imam from a different sect depends on whether this imam’s innovation amounts to disbelief. For the Shiites, this is still applicable. However, it is rather difficult to make a blanket statement about all of them, because of the major differences among the different Shiite sects and even the differences within the same subsect.

Those who commit one or more of the following transgressions among them are not regarded as Muslims:

1. Believe that the Qur’an was altered.

2. Believe that all the Companions but a few apostatized.

3. Say about the Mother of the Believers `A’ishah that which Allah has declared her innocent of.

4. Exaggerate the position of their imams (who were righteous descendants of the Prophet) and elevate them to a position comparable to or above that of the Prophets.

Thus, the Prayer behind those who do one or more of these transgressions would be invalid. However, if one of them does not have an innovation that amounts to disbelief, then the Prayer behind him or her will be valid, though disliked because they still have rulings other than what we have and their form of Prayer is different from ours. Such differences are not like those among the Sunni schools of jurisprudence, which acknowledge the same sources but differ on the interpretation.