Calculate the temperature stability of wavelength demultiplexer

Hi ive been given this question:

A wavelength demultiplexer (DMUX) used in a DWDM communication system has a
channel spacing of 100 GHz. The operating pass band of each channel is 50 GHz. The
transmission wavelength response of the DMUX depends linearly on temperature with
a sensitivity ??/??
= 0.12 nm ℃−1
.If a laser of wavelength 1531 nm is centred in the pass
band of the DMUX, calculate the required temperature stability ∆? of the DMUX.

The given answer is +-1.63 degrees celicius however ive tried to work it out as follows:

Allowed change in wavelength = 3 *10^8 / 25GHz = 0.012 m

Allowed change in temperature = change in wavelength/sensitivity = 0.012/0.12*10^-9 = 10^8

The answer im getting is very wrong, so any help would be much appreciated!