  We employ the accretion disk reflection model RELXILL_NK to test the
spacetime geometry around the stellar-mass black hole in GRS 1915+105. We adopt
the Johannsen metric with the deformation parameters $\alpha_{13}$ and
$\alpha_{22}$, for which the Kerr solution is recovered when $\alpha_{13} =
\alpha_{22} = 0$. We analyze a NuSTAR observation of 2012, obtaining vanishing
and non-vanishing values of the deformation parameters depending on the
astrophysical model adopted. Similar difficulties were not found in our
previous tests with other sources. The results of this work can shed light on
the choice of sources suitable for testing the Kerr metric using X-ray
reflection spectroscopy and on the parts of our reflection models that more
urgently require improvement.
