Neuhäuser et al. (2005a) presented evidence for a sub-stellar, common proper motion companion to GQ Lup, which is almost certainly a planet imaged directly, based on two independent mass estimates by means of theoretical models. The magnitude of GQ Lup b was determined by relative photometry compared to GQ Lup A. During a one-month monitoring of the primary star, we found is to be a variable with K= 6.9 to 7.3 mag (absolute photometry). Then, we re-observed GQ Lup A+b in May 2005, again with VLT/NACO. Due to the monitoring in April 2005, we could predict the exact K-band magnitude range of the primary during the new AO observation. Hence, we could determine a more precise error of the K-band magnitude of GQ Lup b by relative photometry compared to the primary. The new value is consistent with the previous estimate within the error margin, so that also the conclusions in Neuhäuser et al. (2005a,b) about its very low mass remain valid.