Left: derived polarisation degree P (%) against the average S/N of the target for the NOT/ALFOSC imaging polarimetry in the V band. The P values have been corrected for polarisation bias following Plaszczynski et al. (2014). High values of polarisation degree are found only below S/N ∼ 125 (grey region), implying these measurements are untrustworthy. We note that the single epoch of SN 2019szu is also shown in the figure, but is excluded from the analysis because of the low S/N. Right: moon illumination against the SN–Moon separation at the time of the observations. The respective epoch of each SN is shown next to each marker. The epochs that were identified as questionable in comparison to neighbouring observations have been crossed in black. Open markers denote epochs when the Moon was below the horizon. The questionable observations were taken during high lunar illumination, implying that it affects the results, rendering them unreliable.