  Differential magnification is now well-known to distort the spectral energy
distributions of strongly gravitationally lensed galaxies. However, that does
not mean that any distortions are possible. Here I prove an analytic upper
bound to differential magnification effects. For example, a thermal or
sub-thermal CO ladder cannot be made to appear super-thermal just from
gravitational lensing, and the Balmer decrement emission line ratio
H$\alpha$:H$\beta$ cannot reduce below the case B prediction just from
differential magnification. In general, if a physical model of a galaxy
predicts upper and/or lower bounds to an emission line ratio, then those bounds
also apply to the differentially magnified strongly gravitationally lensed
case. This applies not just for velocity-integrated emission lines, but also
for the line emission in any rest-frame velocity interval.
