Illustration that spectra with different flux and different missing bins cannot be simply added. Both spectrum 1 and 2 have 1 s exposure but have different constant expected count rates of 400 and 200 counts s^-1. In spectrum 1 pixel A is missing, in spectrum 2 pixel B. The thick solid line in the total spectrum indicates the predicted number of counts by straightforwardly adding the response matrices for both spectra with equal weights.