Sloan i-band median combined image (T_exp = 9 × 100 s) of the field of GRB 130528A taken with the 10.4 m GTC on Jun. 1, 2013. Circle A & B represents the XRT error circles in 4.1 arcsec (D’Elia et al. 2013) and 1.8 arcsec (Goad et al. 2013) radius, respectively. The ellipse C shows the mm detection beam size by PdBI (see Sect. 2.2 in this paper). It clearly points out the putative host galaxy of GRB 130528A by confirming a mm afterglow with a signal-to-noise ratio of ~4 (at 86.7 GHz). The measured i-band magnitude is 22.87 ± 0.08 in AB system.