Relative minus sign in radiation of gluon jets

I am trying to calculate the cross-section for electron-positron annihilation into a quark-antiquark pair and a gluon. I find that I need a relative minus sign between the two contributing diagrams in order to have $ q_{mu} H^{munu}=0$ with $H^{munu}$ being the hadronic trace and thus conserve the vector current. Can someone tell me where this minus sign comes from? I cannot see where any fermion positions are interchanged here.