Before revision:  I focus on a practically relevant situation in which additional covariates are required for identification but the reduced-form and first-stage regressions are possibly misspecified as a result of neglected heterogeneity in the effects of the instrument .
After revision:  I focus on a practically relevant situation in which additional covariates are required for identification while the reduced-form and first-stage regressions are possibly misspecified as a result of neglected heterogeneity in the effects of the instrument .