The recursive-projection aggregation (RPA) algorithm provides state-of-the-art performance for Reed-Muller (RM) codes. It relies on projecting onto subspaces, decoding projections and voting. This paper explores the error-correction capability of RPA with a limited number of subspaces. By constructing a false vote matrix, it demonstrates that RPA may fail to correct coset error patterns, even when all projections are decoded successfully. These patterns severely degrade the decoding performance due to their small weights. Thus, two rules on the subspace selection are derived to guarantee the error-correction capability of RPA. Simulation results verify that the two rules dominate the decoding performance of RPA.