Normalised HST/STIS spectrum of SN 2018bsz taken at 26.5 d post-maximum brightness (lighter shade) and the spectrum binned to 5 Å (darker shade). Several narrow host galaxy absorption lines of Fe II, Mg I and Mg II as well as Milky Way Mg IIλλ2796, 2803 doublet have been highlighted. Approximate locations of four SLSNe-I absorption bands (UV1: 2650 Å, UV2: 2450 Å, UV3: 2200 Å and UV4: 1950 Å; Quimby et al. 2018) have been marked, but only UV1 is clearly present for SN 2018bsz. In the inset we show a close up of the host galaxy Mg IIλλ2796, 2803 doublet region including a double Gaussian fit to the line profile. The combined equivalent width of the lines is Å. We also show spectrum of PTF12dam for comparison as it is one of the few high-quality HST spectra of SLSNe-I taken at similar epoch to SN 2018bsz. The spectrum was first presented in Quimby et al. (2018) and downloaded from the Open Supernova Catalog (Guillochon et al. 2017).