  We investigate the robustness of the resonance like feature centred at around
a 750 GeV invariant mass in the 13 TeV diphoton data, recently released by the
ATLAS collaboration. We focus on the choice of empirical function used to model
the continuum diphoton background in order to quantify the uncertainties in the
analysis due to this choice. We extend the function chosen by the ATLAS
collaboration to one with two components. By performing a profile likelihood
analysis we find that the local significance of a resonance drops from
$3.9\sigma$ using the ATLAS background function, and a freely-varying width, to
only $2\sigma$ with our own function. We argue that the latter significance is
more realistic, since the former was derived using a function which is fit
almost entirely to the low-energy data, while underfitting in the region around
the resonance.
