diff --git a/library/transformer/src/main/java/com/google/android/exoplayer2/transformer/CompositeAssetLoader.java b/library/transformer/src/main/java/com/google/android/exoplayer2/transformer/CompositeAssetLoader.java
index 34043b947a..efbacc2fc5 100644
--- a/library/transformer/src/main/java/com/google/android/exoplayer2/transformer/CompositeAssetLoader.java
+++ b/library/transformer/src/main/java/com/google/android/exoplayer2/transformer/CompositeAssetLoader.java
@@ -153,7 +153,9 @@ import java.util.concurrent.atomic.AtomicLong;
       sampleConsumersByTrackType.put(trackType, sampleConsumer);
       return sampleConsumer;
     }
-    return checkStateNotNull(sampleConsumersByTrackType.get(trackType));
+    return checkStateNotNull(
+        sampleConsumersByTrackType.get(trackType),
+        "The preceding MediaItem does not contain any track of type " + trackType);
   }
 
   @Override