diff --git a/src/mlpack/core/optimizers/sgd/test_function.cpp b/src/mlpack/core/optimizers/sgd/test_function.cpp
index f3f4ce92fc..8415d98613 100644
--- a/src/mlpack/core/optimizers/sgd/test_function.cpp
+++ b/src/mlpack/core/optimizers/sgd/test_function.cpp
@@ -52,7 +52,7 @@ double SGDTestFunction::Evaluate(const arma::mat& coordinates,
 {
   double objective = 0;
 
-  for (int i = begin; i < begin + batchSize; i++)
+  for (size_t i = begin; i < begin + batchSize; i++)
   {
     switch (visitationOrder(i))
     {