  We present imaging polarimetry of the superluminous supernova SN 2015bn,
obtained over nine epochs between $-$20 and $+$46 days with the Nordic Optical
Telescope. This was a nearby, slowly-evolving Type I superluminous supernova
that has been studied extensively and for which two epochs of
spectropolarimetry are also available. Based on field stars, we determine the
interstellar polarisation in the Galaxy to be negligible. The polarisation of
SN 2015bn shows a statistically significant increase during the last epochs,
confirming previous findings. Our well-sampled imaging polarimetry series
allows us to determine that this increase (from $\sim 0.54\%$ to $\gtrsim
1.10\%$) coincides in time with rapid changes that took place in the optical
spectrum. We conclude that the supernova underwent a `phase transition' at
around $+$20 days, when the photospheric emission shifted from an outer layer,
dominated by natal C and O, to a more aspherical inner core, dominated by
freshly nucleosynthesized material. This two-layered model might account for
the characteristic appearance and properties of Type I superluminous
supernovae.
