Fuzzy regression-discontinuity evaluations of college remediation often find negative and null estimates of local average treatments effects (LATEs), but with substantial heterogeneity. We find that a remedial quantitative skills course at Wellesley College has a modestly positive LATE on participation in mathematically intensive fields of study--including the sciences, mathematics, and economics courses. Yet, LATEs are a weighted average of average causal effects (at the passing cutoff) in two principal strata: students who voluntarily comply with remediation, and those who are coerced to comply after scoring below the cutoff on an optional retest. In the retest sample, we show that average causal effects are close to zero among (1) coerced compliers, and (2) never-takers. By implication, there are even larger effects among a smaller group of voluntary compliers at the cutoff. The results help interpret the mixed findings in the literature, in which compliance varies widely, and demonstrate methods for assessing external validity in fuzzy-discontinuity designs.