diff --git a/_pyceres/core/problem.h b/_pyceres/core/problem.h
index be37cec..ccee5bf 100644
--- a/_pyceres/core/problem.h
+++ b/_pyceres/core/problem.h
@@ -11,6 +11,7 @@ namespace py = pybind11;
 
 namespace {
 
+// Set residual blocks for Ceres::Problem::EvaluateOptions
 void SetResidualBlocks(
     ceres::Problem::EvaluateOptions& self,
     std::vector<ResidualBlockIDWrapper>& residual_block_ids) {