diff --git a/library/core/src/main/java/com/google/android/exoplayer2/source/ConcatenatingMediaSource.java b/library/core/src/main/java/com/google/android/exoplayer2/source/ConcatenatingMediaSource.java
index aaf2b6fef1..20df2a8ab5 100644
--- a/library/core/src/main/java/com/google/android/exoplayer2/source/ConcatenatingMediaSource.java
+++ b/library/core/src/main/java/com/google/android/exoplayer2/source/ConcatenatingMediaSource.java
@@ -392,17 +392,16 @@ public class ConcatenatingMediaSource extends CompositeMediaSource<MediaSourceHo
   public final MediaPeriod createPeriod(MediaPeriodId id, Allocator allocator) {
     int mediaSourceHolderIndex = findMediaSourceHolderByPeriodIndex(id.periodIndex);
     MediaSourceHolder holder = mediaSourceHolders.get(mediaSourceHolderIndex);
-    MediaPeriodId idInSource =
-        id.copyWithPeriodIndex(id.periodIndex - holder.firstPeriodIndexInChild);
-    DeferredMediaPeriod mediaPeriod =
-        new DeferredMediaPeriod(holder.mediaSource, idInSource, allocator);
+    DeferredMediaPeriod mediaPeriod = new DeferredMediaPeriod(holder.mediaSource, id, allocator);
     mediaSourceByMediaPeriod.put(mediaPeriod, holder);
     holder.activeMediaPeriods.add(mediaPeriod);
     if (!holder.hasStartedPreparing) {
       holder.hasStartedPreparing = true;
       prepareChildSource(holder, holder.mediaSource);
     } else if (holder.isPrepared) {
-      mediaPeriod.createPeriod();
+      MediaPeriodId idInSource =
+          id.copyWithPeriodIndex(id.periodIndex - holder.firstPeriodIndexInChild);
+      mediaPeriod.createPeriod(idInSource);
     }
     return mediaPeriod;
   }
@@ -599,7 +598,10 @@ public class ConcatenatingMediaSource extends CompositeMediaSource<MediaSourceHo
       for (int i = 0; i < mediaSourceHolder.activeMediaPeriods.size(); i++) {
         DeferredMediaPeriod deferredMediaPeriod = mediaSourceHolder.activeMediaPeriods.get(i);
         deferredMediaPeriod.setDefaultPreparePositionUs(defaultPeriodPositionUs);
-        deferredMediaPeriod.createPeriod();
+        MediaPeriodId idInSource =
+            deferredMediaPeriod.id.copyWithPeriodIndex(
+                deferredMediaPeriod.id.periodIndex - mediaSourceHolder.firstPeriodIndexInChild);
+        deferredMediaPeriod.createPeriod(idInSource);
       }
       mediaSourceHolder.isPrepared = true;
     }
diff --git a/library/core/src/main/java/com/google/android/exoplayer2/source/DeferredMediaPeriod.java b/library/core/src/main/java/com/google/android/exoplayer2/source/DeferredMediaPeriod.java
index 229043b127..0dc4b4c888 100644
--- a/library/core/src/main/java/com/google/android/exoplayer2/source/DeferredMediaPeriod.java
+++ b/library/core/src/main/java/com/google/android/exoplayer2/source/DeferredMediaPeriod.java
@@ -40,7 +40,9 @@ public final class DeferredMediaPeriod implements MediaPeriod, MediaPeriod.Callb
     void onPrepareError(MediaPeriodId mediaPeriodId, IOException exception);
   }
 
+  /** The {@link MediaSource} which will create the actual media period. */
   public final MediaSource mediaSource;
+  /** The {@link MediaPeriodId} used to create the deferred media period. */
   public final MediaPeriodId id;
 
   private final Allocator allocator;
@@ -56,7 +58,7 @@ public final class DeferredMediaPeriod implements MediaPeriod, MediaPeriod.Callb
    * Creates a new deferred media period.
    *
    * @param mediaSource The media source to wrap.
-   * @param id The identifier for the media period to create when {@link #createPeriod()} is called.
+   * @param id The identifier used to create the deferred media period.
    * @param allocator The allocator used to create the media period.
    */
   public DeferredMediaPeriod(MediaSource mediaSource, MediaPeriodId id, Allocator allocator) {
@@ -100,8 +102,10 @@ public final class DeferredMediaPeriod implements MediaPeriod, MediaPeriod.Callb
    * Calls {@link MediaSource#createPeriod(MediaPeriodId, Allocator)} on the wrapped source then
    * prepares it if {@link #prepare(Callback, long)} has been called. Call {@link #releasePeriod()}
    * to release the period.
+   *
+   * @param id The identifier that should be used to create the media period from the media source.
    */
-  public void createPeriod() {
+  public void createPeriod(MediaPeriodId id) {
     mediaPeriod = mediaSource.createPeriod(id, allocator);
     if (callback != null) {
       mediaPeriod.prepare(this, preparePositionUs);
diff --git a/library/core/src/main/java/com/google/android/exoplayer2/source/ads/AdsMediaSource.java b/library/core/src/main/java/com/google/android/exoplayer2/source/ads/AdsMediaSource.java
index e3ec73b95e..fed3d1526e 100644
--- a/library/core/src/main/java/com/google/android/exoplayer2/source/ads/AdsMediaSource.java
+++ b/library/core/src/main/java/com/google/android/exoplayer2/source/ads/AdsMediaSource.java
@@ -170,6 +170,10 @@ public final class AdsMediaSource extends CompositeMediaSource<MediaPeriodId> {
 
   }
 
+  // Used to identify the content "child" source for CompositeMediaSource.
+  private static final MediaPeriodId DUMMY_CONTENT_MEDIA_PERIOD_ID =
+      new MediaPeriodId(/* periodIndex= */ 0);
+
   private final MediaSource contentMediaSource;
   private final MediaSourceFactory adMediaSourceFactory;
   private final AdsLoader adsLoader;
@@ -314,7 +318,7 @@ public final class AdsMediaSource extends CompositeMediaSource<MediaPeriodId> {
     Assertions.checkArgument(isTopLevelSource);
     final ComponentListener componentListener = new ComponentListener();
     this.componentListener = componentListener;
-    prepareChildSource(new MediaPeriodId(/* periodIndex= */ 0), contentMediaSource);
+    prepareChildSource(DUMMY_CONTENT_MEDIA_PERIOD_ID, contentMediaSource);
     mainHandler.post(new Runnable() {
       @Override
       public void run() {
@@ -344,16 +348,14 @@ public final class AdsMediaSource extends CompositeMediaSource<MediaPeriodId> {
         prepareChildSource(id, adMediaSource);
       }
       MediaSource mediaSource = adGroupMediaSources[adGroupIndex][adIndexInAdGroup];
-      DeferredMediaPeriod deferredMediaPeriod =
-          new DeferredMediaPeriod(
-              mediaSource,
-              new MediaPeriodId(/* periodIndex= */ 0, id.windowSequenceNumber),
-              allocator);
+      DeferredMediaPeriod deferredMediaPeriod = new DeferredMediaPeriod(mediaSource, id, allocator);
       deferredMediaPeriod.setPrepareErrorListener(
           new AdPrepareErrorListener(adUri, adGroupIndex, adIndexInAdGroup));
       List<DeferredMediaPeriod> mediaPeriods = deferredMediaPeriodByAdMediaSource.get(mediaSource);
       if (mediaPeriods == null) {
-        deferredMediaPeriod.createPeriod();
+        MediaPeriodId adSourceMediaPeriodId =
+            new MediaPeriodId(/* periodIndex= */ 0, id.windowSequenceNumber);
+        deferredMediaPeriod.createPeriod(adSourceMediaPeriodId);
       } else {
         // Keep track of the deferred media period so it can be populated with the real media period
         // when the source's info becomes available.
@@ -362,7 +364,7 @@ public final class AdsMediaSource extends CompositeMediaSource<MediaPeriodId> {
       return deferredMediaPeriod;
     } else {
       DeferredMediaPeriod mediaPeriod = new DeferredMediaPeriod(contentMediaSource, id, allocator);
-      mediaPeriod.createPeriod();
+      mediaPeriod.createPeriod(id);
       return mediaPeriod;
     }
   }
@@ -415,8 +417,8 @@ public final class AdsMediaSource extends CompositeMediaSource<MediaPeriodId> {
   @Override
   protected @Nullable MediaPeriodId getMediaPeriodIdForChildMediaPeriodId(
       MediaPeriodId childId, MediaPeriodId mediaPeriodId) {
-    // The child id for the content period is just a dummy without window sequence number. That's
-    // why we need to forward the reported mediaPeriodId in this case.
+    // The child id for the content period is just DUMMY_CONTENT_MEDIA_PERIOD_ID. That's why we need
+    // to forward the reported mediaPeriodId in this case.
     return childId.isAd() ? childId : mediaPeriodId;
   }
 
@@ -443,12 +445,14 @@ public final class AdsMediaSource extends CompositeMediaSource<MediaPeriodId> {
       int adIndexInAdGroup, Timeline timeline) {
     Assertions.checkArgument(timeline.getPeriodCount() == 1);
     adDurationsUs[adGroupIndex][adIndexInAdGroup] = timeline.getPeriod(0, period).getDurationUs();
-    if (deferredMediaPeriodByAdMediaSource.containsKey(mediaSource)) {
-      List<DeferredMediaPeriod> mediaPeriods = deferredMediaPeriodByAdMediaSource.get(mediaSource);
+    List<DeferredMediaPeriod> mediaPeriods = deferredMediaPeriodByAdMediaSource.remove(mediaSource);
+    if (mediaPeriods != null) {
       for (int i = 0; i < mediaPeriods.size(); i++) {
-        mediaPeriods.get(i).createPeriod();
+        DeferredMediaPeriod mediaPeriod = mediaPeriods.get(i);
+        MediaPeriodId adSourceMediaPeriodId =
+            new MediaPeriodId(/* periodIndex= */ 0, mediaPeriod.id.windowSequenceNumber);
+        mediaPeriod.createPeriod(adSourceMediaPeriodId);
       }
-      deferredMediaPeriodByAdMediaSource.remove(mediaSource);
     }
     maybeUpdateSourceInfo();
   }