  A chiral chemical potential present in the early universe can source helical
hypermagnetic fields through the chiral plasma instability. If these
hypermagnetic fields survive until the electroweak phase transition, they
source a contribution to the baryon asymmetry of the universe. In this letter,
we demonstrate that lepton flavour asymmetries above $|\mu|/T \sim 9 \times
10^{-3}$ trigger this mechanism even for vanishing total lepton number. This
excludes the possibility of such large lepton flavour asymmetries present at
temperatures above $10^6$ GeV, setting a constraint which is about two orders
of magnitude stronger than the current CMB and BBN limits.
