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 ff95afb1f6..d176b1905c 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;