diff --git a/library/common/src/main/java/com/google/android/exoplayer2/MediaItem.java b/library/common/src/main/java/com/google/android/exoplayer2/MediaItem.java
index ce6adde1b8..f891bb9662 100644
--- a/library/common/src/main/java/com/google/android/exoplayer2/MediaItem.java
+++ b/library/common/src/main/java/com/google/android/exoplayer2/MediaItem.java
@@ -1189,8 +1189,8 @@ public final class MediaItem implements Bundleable {
 
     /** The {@link Uri} to the subtitle file. */
     public final Uri uri;
-    /** The MIME type. */
-    public final String mimeType;
+    /** The optional MIME type of the subtitle file, or {@code null} if unspecified. */
+    @Nullable public final String mimeType;
     /** The language. */
     @Nullable public final String language;
     /** The selection flags. */
@@ -1261,7 +1261,7 @@ public final class MediaItem implements Bundleable {
       Subtitle other = (Subtitle) obj;
 
       return uri.equals(other.uri)
-          && mimeType.equals(other.mimeType)
+          && Util.areEqual(mimeType, other.mimeType)
           && Util.areEqual(language, other.language)
           && selectionFlags == other.selectionFlags
           && roleFlags == other.roleFlags
@@ -1271,7 +1271,7 @@ public final class MediaItem implements Bundleable {
     @Override
     public int hashCode() {
       int result = uri.hashCode();
-      result = 31 * result + mimeType.hashCode();
+      result = 31 * result + (mimeType == null ? 0 : mimeType.hashCode());
       result = 31 * result + (language == null ? 0 : language.hashCode());
       result = 31 * result + selectionFlags;
       result = 31 * result + roleFlags;
diff --git a/library/common/src/main/java/com/google/android/exoplayer2/util/MimeTypes.java b/library/common/src/main/java/com/google/android/exoplayer2/util/MimeTypes.java
index 3e82accd71..493485133a 100644
--- a/library/common/src/main/java/com/google/android/exoplayer2/util/MimeTypes.java
+++ b/library/common/src/main/java/com/google/android/exoplayer2/util/MimeTypes.java
@@ -98,6 +98,8 @@ public final class MimeTypes {
 
   public static final String TEXT_EXOPLAYER_CUES = BASE_TYPE_TEXT + "/x-exoplayer-cues";
 
+  public static final String TEXT_UNKNOWN = BASE_TYPE_TEXT + "/x-unknown";
+
   // application/ MIME types
 
   public static final String APPLICATION_MP4 = BASE_TYPE_APPLICATION + "/mp4";
diff --git a/library/core/src/main/java/com/google/android/exoplayer2/source/DefaultMediaSourceFactory.java b/library/core/src/main/java/com/google/android/exoplayer2/source/DefaultMediaSourceFactory.java
index 72039bce80..a5ffff4998 100644
--- a/library/core/src/main/java/com/google/android/exoplayer2/source/DefaultMediaSourceFactory.java
+++ b/library/core/src/main/java/com/google/android/exoplayer2/source/DefaultMediaSourceFactory.java
@@ -381,7 +381,7 @@ public final class DefaultMediaSourceFactory implements MediaSourceFactory {
       mediaSources[0] = mediaSource;
       for (int i = 0; i < subtitles.size(); i++) {
         if (useProgressiveMediaSourceForSubtitles
-            && subtitles.get(i).mimeType.equals(MimeTypes.TEXT_VTT)) {
+            && MimeTypes.TEXT_VTT.equals(subtitles.get(i).mimeType)) {
           int index = i;
           ProgressiveMediaSource.Factory progressiveMediaSourceFactory =
               new ProgressiveMediaSource.Factory(
diff --git a/library/core/src/main/java/com/google/android/exoplayer2/source/SingleSampleMediaSource.java b/library/core/src/main/java/com/google/android/exoplayer2/source/SingleSampleMediaSource.java
index c0d34d0ff3..887ce26463 100644
--- a/library/core/src/main/java/com/google/android/exoplayer2/source/SingleSampleMediaSource.java
+++ b/library/core/src/main/java/com/google/android/exoplayer2/source/SingleSampleMediaSource.java
@@ -16,6 +16,7 @@
 package com.google.android.exoplayer2.source;
 
 import static com.google.android.exoplayer2.util.Assertions.checkNotNull;
+import static com.google.common.base.MoreObjects.firstNonNull;
 
 import android.net.Uri;
 import androidx.annotation.Nullable;
@@ -28,6 +29,7 @@ import com.google.android.exoplayer2.upstream.DataSpec;
 import com.google.android.exoplayer2.upstream.DefaultLoadErrorHandlingPolicy;
 import com.google.android.exoplayer2.upstream.LoadErrorHandlingPolicy;
 import com.google.android.exoplayer2.upstream.TransferListener;
+import com.google.android.exoplayer2.util.MimeTypes;
 import java.util.Collections;
 
 /**
@@ -163,7 +165,7 @@ public final class SingleSampleMediaSource extends BaseMediaSource {
     format =
         new Format.Builder()
             .setId(trackId)
-            .setSampleMimeType(subtitle.mimeType)
+            .setSampleMimeType(firstNonNull(subtitle.mimeType, MimeTypes.TEXT_UNKNOWN))
             .setLanguage(subtitle.language)
             .setSelectionFlags(subtitle.selectionFlags)
             .setRoleFlags(subtitle.roleFlags)