diff --git a/src/mlpack/methods/ann/ffn_impl.hpp b/src/mlpack/methods/ann/ffn_impl.hpp
index 10c5c215a8..551206051a 100644
--- a/src/mlpack/methods/ann/ffn_impl.hpp
+++ b/src/mlpack/methods/ann/ffn_impl.hpp
@@ -232,7 +232,7 @@ void FFN<
   }
   
   // Perform the backward pass.
-  networkDelta.set_size(predictors.n_rows, batchSize);
+  networkDelta.set_size(predictors.n_rows, 1);
   network.Backward(networkOutput, error, gradients, networkDelta);
 }