diff --git a/src/types/Batch.hpp b/src/types/Batch.hpp
index 17293a4..595015d 100644
--- a/src/types/Batch.hpp
+++ b/src/types/Batch.hpp
@@ -132,7 +132,7 @@ class Batch {
 	 * @param index_ Index of the sample from the batch.
 	 * @return Sample number.
 	 */
-	std::vector <size_t> indices(size_t index_) {
+	size_t indices(size_t index_) {
 		return sample_indices[index_];
 	}