In 2015, Fong, Finkelstein, Jaeger, Diaz, and Broek reported the findings from an independent evaluation of the Expository Reading and Writing Course (ERWC). The evaluation found positive and statistically significant effects of the ERWC on student achievement. Due to some concerns that the previously reported results in Fong et al. (2015) did not account for the possibility that students who were taught by the same teacher may have correlated error terms, an additional ordinary least squares (OLS) regression analysis was conducted using cluster-robust standard errors that allow for clustering on the teacher. The results of this additional analysis are reported in this addendum. The previously reported OLS regression results had used cluster-robust standard errors that allowed for clustering on the student. The newly reported regression analysis in this addendum only clusters on the teacher, and not on both the teacher and the student, since previous literature has recommended only clustering at the highest level (see, for instance, Cameron & Miller, 2015; Bertrand, Duflo, & Mullainathan, 2004). [For the full report, "Evaluation of the Expository Reading and Writing Course: Findings from the Investing in Innovation Development Grant," see ED559522.]