  Femtolensing of gamma ray bursts (GRBs) has been put forward as an exciting
possibility to probe exotic astrophysical objects with masses below $10^{-13}$
solar masses such as small primordial black holes or ultra-compact dark matter
minihalos, made up for instance of QCD axions. In this paper we critically
review this idea, properly taking into account the extended nature of the
source as well as wave optics effects. We demonstrate that most GRBs are
inappropriate for femtolensing searches due to their large sizes. This removes
the previous femtolensing bounds on primordial black holes, implying that vast
regions of parameter space for primordial black hole dark matter are not
robustly constrained. Still, we entertain the possibility that a small fraction
of GRBs, characterized by fast variability can have smaller sizes and be
useful. However, a large number of such bursts would need to be observed to
achieve meaningful constraints. We study the sensitivity of future observations
as a function of the number of detected GRBs and of the size of the emission
region.
