90 Litteus of Gemelli said: "'If the blind lead the blind, both shall fall into the ditch.' Since, therefore, it is clear that heretics can give no light to any one, as being blind themselves, therefore their baptism is invalid."
91. Neither do we say that it is valid for salvation so long as they are heretics, just as it is of no value to those murderers of whom we spoke, so long as they hate their brethren: for they also themselves are in darkness, and if any one follows them they fall together into the ditch; and yet it does not follow that they either have not baptism or are unable to confer it.
Source: On Baptism, Against the Donatists (New Advent)