The McWhorter model explains 1/f noise as an addition of generation-recombination spectra. Here we investigate whether addition of spectra is allowed. A condition for additivity is derived. If that condition is not met, then the GR spectra do not add together but they mix; resulting in one Lorentzian with tL given by 1/tL=¿1/ti. A 1/f-like spectrum appears at very low frequencies. A numerical estimate for the 1/f noise parameter a follows from our analysis. The disagreement with experimental values found with MOSTs makes it very unlikely for the McWhorter model to be correct.