In cfhtIsrTask.py the readout noise for amp A and amp B is set from the values stored in the raw image fits header identified by the keywords RDNOISEA and RDNOISEB. Typically the noise level is ~5 electrons. For some old CFHT images acquired in 2004, RDNOISEA/B is equal to 65535. In these cases, the parameter identified by the keyword RDNOISE is nevertheless correct.
cfhtIsrTask.py should be modified to use RDNOISE instead of RDNOISEA/B when this situation happens.