Hi Jack,
This should not happen under normal conditions with linear images. When the MRS noise evaluation routine fails, it is usually because the image has no noise
![Grin ;D](http://pixinsight.com/forum/Smileys/default/grin.gif)
. This includes images with no signal (e.g. a black image) and very smooth
nonlinear images.
I am very interested in taking a look at the image that caused this problem. Can you upload it? Thanks!