diff --git a/libraries/transformer/src/androidTest/java/androidx/media3/transformer/TransformerGapsTest.java b/libraries/transformer/src/androidTest/java/androidx/media3/transformer/TransformerGapsTest.java
index 7c4f9918c3..235047a9c8 100644
--- a/libraries/transformer/src/androidTest/java/androidx/media3/transformer/TransformerGapsTest.java
+++ b/libraries/transformer/src/androidTest/java/androidx/media3/transformer/TransformerGapsTest.java
@@ -135,29 +135,6 @@ public class TransformerGapsTest {
         .isEqualTo(2 * MP4_ASSET.videoFrameCount + expectedBlankFrames);
   }
 
-  // TODO: b/391111085 - Change test when gaps at the start of the sequence are supported.
-  @Test
-  public void export_withTwoVideoOnlyMediaItemsAndGapAtStart_throws() {
-    Transformer transformer = new Transformer.Builder(context).build();
-    Composition composition =
-        new Composition.Builder(
-                new EditedMediaItemSequence.Builder()
-                    .addGap(/* durationUs= */ 1_000_000)
-                    .addItem(VIDEO_ONLY_MEDIA_ITEM)
-                    .addItem(VIDEO_ONLY_MEDIA_ITEM)
-                    .build())
-            .build();
-    TransformerAndroidTestRunner transformerAndroidTestRunner =
-        new TransformerAndroidTestRunner.Builder(context, transformer).build();
-
-    // An IllegalStateException is thrown instead of an ExportException because the exception is
-    // thrown very early in the setup phase and its not caught.
-    // TODO: b/391111085 - Throw exception when the sequence without force audio/video flag is
-    // built.
-    assertThrows(
-        IllegalStateException.class, () -> transformerAndroidTestRunner.run(testId, composition));
-  }
-
   @Test
   public void export_withTwoVideoOnlyMediaItemsAndGapInMiddle_insertsBlankFramesForGap()
       throws Exception {
@@ -224,27 +201,15 @@ public class TransformerGapsTest {
         .isEqualTo(2 * MP4_ASSET.videoFrameCount + expectedBlankFrames);
   }
 
-  // TODO: b/391111085 - Change test when gaps at the start of the sequence are supported.
   @Test
-  public void export_withTwoMediaItemsAndGapAtStart_throws() {
-    Transformer transformer = new Transformer.Builder(context).build();
-    Composition composition =
-        new Composition.Builder(
-                new EditedMediaItemSequence.Builder()
-                    .addGap(/* durationUs= */ 1_000_000)
-                    .addItem(AUDIO_VIDEO_MEDIA_ITEM)
-                    .addItem(AUDIO_VIDEO_MEDIA_ITEM)
-                    .build())
-            .build();
-    TransformerAndroidTestRunner transformerAndroidTestRunner =
-        new TransformerAndroidTestRunner.Builder(context, transformer).build();
+  public void buildSequence_withTwoMediaItemsAndGapAtStart_throws() {
+    EditedMediaItemSequence.Builder sequenceBuilder =
+        new EditedMediaItemSequence.Builder()
+            .addGap(/* durationUs= */ 1_000_000)
+            .addItem(AUDIO_VIDEO_MEDIA_ITEM)
+            .addItem(AUDIO_VIDEO_MEDIA_ITEM);
 
-    // An IllegalStateException is thrown instead of an ExportException because the exception is
-    // thrown very early in the setup phase and its not caught.
-    // TODO: b/391111085 - Throw exception when the sequence without force audio/video flag is
-    // built.
-    assertThrows(
-        IllegalStateException.class, () -> transformerAndroidTestRunner.run(testId, composition));
+    assertThrows(IllegalArgumentException.class, sequenceBuilder::build);
   }
 
   @Test
diff --git a/libraries/transformer/src/main/java/androidx/media3/transformer/EditedMediaItemSequence.java b/libraries/transformer/src/main/java/androidx/media3/transformer/EditedMediaItemSequence.java
index ffa9d7819d..02b9bf405b 100644
--- a/libraries/transformer/src/main/java/androidx/media3/transformer/EditedMediaItemSequence.java
+++ b/libraries/transformer/src/main/java/androidx/media3/transformer/EditedMediaItemSequence.java
@@ -98,6 +98,9 @@ public final class EditedMediaItemSequence {
      *
      * <p>A gap is a period of time with no media.
      *
+     * <p>If the gap is at the start of the sequence then {@linkplain #setForceAudioTrack(boolean)
+     * force audio track} flag must be set to force silent audio.
+     *
      * <p>Gaps at the start of the sequence are not supported if the sequence has video.
      *
      * @param durationUs The duration of the gap, in milliseconds.
@@ -230,6 +233,9 @@ public final class EditedMediaItemSequence {
     this.editedMediaItems = builder.items.build();
     checkArgument(
         !editedMediaItems.isEmpty(), "The sequence must contain at least one EditedMediaItem.");
+    checkArgument(
+        !editedMediaItems.get(0).isGap() || builder.forceAudioTrack,
+        "If the first item in the sequence is a Gap, then forceAudioTrack flag must be set");
     this.isLooping = builder.isLooping;
     this.forceAudioTrack = builder.forceAudioTrack;
   }