diff --git a/RELEASENOTES.md b/RELEASENOTES.md
index 535996d0b7..4edc5a5de1 100644
--- a/RELEASENOTES.md
+++ b/RELEASENOTES.md
@@ -11,8 +11,15 @@
 * Rename `MediaCodecRenderer.onOutputFormatChanged` to
   `MediaCodecRenderer.onOutputMediaFormatChanged`, further
   clarifying the distinction between `Format` and `MediaFormat`.
-* Downloads: Merge downloads in `SegmentDownloader` to improve overall download
-  speed ([#5978](https://github.com/google/ExoPlayer/issues/5978)).
+* Downloads:
+  * Merge downloads in `SegmentDownloader` to improve overall download
+    speed ([#5978](https://github.com/google/ExoPlayer/issues/5978)).
+  * Fix handling of network transitions in `RequirementsWatcher`
+    ([#6733](https://github.com/google/ExoPlayer/issues/6733)). Incorrect
+    handling could previously cause downloads to be paused when they should have
+    been able to proceed.
+  * Fix `DownloadHelper.createMediaSource` to use `customCacheKey` when creating
+    `ProgressiveMediaSource` instances.
 * In MP4 streams, store the Android capture frame rate only in
   `Format.metadata`. `Format.frameRate` now stores the calculated frame rate.
 * Add `play` and `pause` methods to `Player`.
@@ -28,10 +35,6 @@
   developers to handle data that's neither UTF-8 nor ISO-8859-1
   ([#6753](https://github.com/google/ExoPlayer/issues/6753)).
 * Add playlist API ([#6161](https://github.com/google/ExoPlayer/issues/6161)).
-* Fix handling of network transitions in `RequirementsWatcher`
-  ([#6733](https://github.com/google/ExoPlayer/issues/6733)). Incorrect handling
-  could previously cause downloads to be paused when they should have been able
-  to proceed.
 * Fix handling of E-AC-3 streams that contain AC-3 syncframes
   ([#6602](https://github.com/google/ExoPlayer/issues/6602)).
 * Fix playback of TrueHD streams in Matroska
diff --git a/library/core/src/main/java/com/google/android/exoplayer2/offline/DownloadHelper.java b/library/core/src/main/java/com/google/android/exoplayer2/offline/DownloadHelper.java
index 1641b2aef6..02a6ad2cb9 100644
--- a/library/core/src/main/java/com/google/android/exoplayer2/offline/DownloadHelper.java
+++ b/library/core/src/main/java/com/google/android/exoplayer2/offline/DownloadHelper.java
@@ -448,6 +448,7 @@ public final class DownloadHelper {
         break;
       case DownloadRequest.TYPE_PROGRESSIVE:
         return new ProgressiveMediaSource.Factory(dataSourceFactory)
+            .setCustomCacheKey(downloadRequest.customCacheKey)
             .createMediaSource(downloadRequest.uri);
       default:
         throw new IllegalStateException("Unsupported type: " + downloadRequest.type);
diff --git a/library/core/src/main/java/com/google/android/exoplayer2/source/ProgressiveMediaSource.java b/library/core/src/main/java/com/google/android/exoplayer2/source/ProgressiveMediaSource.java
index f05b576acb..512fbce4a2 100644
--- a/library/core/src/main/java/com/google/android/exoplayer2/source/ProgressiveMediaSource.java
+++ b/library/core/src/main/java/com/google/android/exoplayer2/source/ProgressiveMediaSource.java
@@ -111,7 +111,7 @@ public final class ProgressiveMediaSource extends BaseMediaSource
      * @return This factory, for convenience.
      * @throws IllegalStateException If {@link #createMediaSource(Uri)} has already been called.
      */
-    public Factory setCustomCacheKey(String customCacheKey) {
+    public Factory setCustomCacheKey(@Nullable String customCacheKey) {
       Assertions.checkState(!isCreateCalled);
       this.customCacheKey = customCacheKey;
       return this;