diff --git a/unsupported/Eigen/CXX11/src/Tensor/TensorReduction.h b/unsupported/Eigen/CXX11/src/Tensor/TensorReduction.h
index 3f1864fa7..fe1dc22ee 100644
--- a/unsupported/Eigen/CXX11/src/Tensor/TensorReduction.h
+++ b/unsupported/Eigen/CXX11/src/Tensor/TensorReduction.h
@@ -298,7 +298,7 @@ struct FullReducer<Self, Op, ThreadPoolDevice, true> {
       *output = reducer.finalize(reducer.initialize());
       return;
     }
-    const int num_threads = device.numThreads();
+    const std::size_t num_threads = device.numThreads();
     if (num_threads == 1) {
       *output = InnerMostDimReducer<Self, Op, true>::reduce(self, 0, num_coeffs, reducer);
       return;