We study here some consequences of the nonlinearities of the electromagneti
c field acting as a source of Einstein's equations on the propagation of ph
otons. We restrict to the particular case of a 'regular black hole', and sh
ow that there exist singularities in the effective geometry. These singular
ities may be hidden behind a horizon or be naked, according to the value of
a parameter. Some unusual properties of this solution are also analysed.