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We employ the accretion disk reflection model RELXILL_NK to test thespacetime geometry around the stellar-mass black hole in GRS 1915+105. We adoptthe 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 vanishingand non-vanishing values of the deformation parameters depending on theastrophysical model adopted. Similar difficulties were not found in ourprevious tests with other sources. The results of this work can shed light onthe choice of sources suitable for testing the Kerr metric using X-rayreflection spectroscopy and on the parts of our reflection models that moreurgently require improvement.

About the Kerr nature of the stellar-mass black hole in GRS 1915+105