We report on the results of the 2+1 flavour QCD simulations at nonzero isospin chemical potential performed at half the physical light quark mass. At low temperatures and large isospin chemical potential Bose-Einstein Condensation (BEC) occurs, creating a pion condensed phase, separated from the hadronic and quark-gluon plasma phases by the BEC transition line. For physical quark masses, the section of this line between the hadronic and BEC phases was found to be almost perfectly vertical, i.e. aligned with the temperature axis. We show that for lighter than physical pions, this section remains vertical, and approaches the axis of vanishing chemical potential linearly with the pion mass, giving a prediction of the phase diagram in the chiral limit.