diff --git a/include/ceres/dynamic_cost_function_to_functor.h b/include/ceres/dynamic_cost_function_to_functor.h
index 31280dc..2c65059 100644
--- a/include/ceres/dynamic_cost_function_to_functor.h
+++ b/include/ceres/dynamic_cost_function_to_functor.h
@@ -116,7 +116,7 @@
   bool operator()(JetT const* const* inputs, JetT* output) const {
     const std::vector<int32>& parameter_block_sizes =
         cost_function_->parameter_block_sizes();
-    const int num_parameter_blocks = parameter_block_sizes.size();
+    const int num_parameter_blocks = static_cast<int>(parameter_block_sizes.size());
     const int num_residuals = cost_function_->num_residuals();
     const int num_parameters = std::accumulate(parameter_block_sizes.begin(),
                                                parameter_block_sizes.end(), 0);