remaining faithful to my lazy approach :-) I add to the discussion rather than checking in other environments. Explaining how I discovered the point may help: I was trying to generalise a script written by somebody else for cases when both matrices involved were bound to be non-scalars, to include a case where the first one can collapse to a scalar (for instance because of restrictions that reduce the parameter space from many to one dimenion). So the problem arises when you have an object that can either be a matrix or a scalar. My view is that the code should work in both cases. Of course, the easy way is writing X'*Y! Stefano