Quark Mass Corrections to the Perturbative Thrust and its Effect on the determination of $alpha_s$

We consider the effects of quark masses to the perturbative thrust in $e^+e^-$ annihilation. In particular we show that perturbative power corrections resulting from non-zero quark masses considerably alters the size of the non-perturbative power corrections and consequently, significantly changes the fitted value of $alpha_s$.