diff --git a/library/core/src/main/java/com/google/android/exoplayer2/source/ShuffleOrder.java b/library/core/src/main/java/com/google/android/exoplayer2/source/ShuffleOrder.java
index e8f804fbe0..9c4e502dad 100644
--- a/library/core/src/main/java/com/google/android/exoplayer2/source/ShuffleOrder.java
+++ b/library/core/src/main/java/com/google/android/exoplayer2/source/ShuffleOrder.java
@@ -23,6 +23,9 @@ import java.util.Random;
  * Shuffled order of indices.
  *
  * <p>The shuffle order must be immutable to ensure thread safety.
+ *
+ * <p>The order must be consistent when traversed both {@linkplain #getNextIndex(int) forwards} and
+ * {@linkplain #getPreviousIndex(int) backwards}.
  */
 public interface ShuffleOrder {