diff --git a/app/src/main/java/org/schabi/newpipe/database/playlist/PlaylistLocalItem.java b/app/src/main/java/org/schabi/newpipe/database/playlist/PlaylistLocalItem.java
index 8b01a636a..352d12d6b 100644
--- a/app/src/main/java/org/schabi/newpipe/database/playlist/PlaylistLocalItem.java
+++ b/app/src/main/java/org/schabi/newpipe/database/playlist/PlaylistLocalItem.java
@@ -13,13 +13,34 @@ public interface PlaylistLocalItem extends LocalItem {
 
     long getDisplayIndex();
 
+    /**
+     * Merge localPlaylists and remotePlaylists by the display index.
+     * If two items have the same display index, sort them in {@code CASE_INSENSITIVE_ORDER}.
+     *
+     * @param localPlaylists  local playlists in the display index order
+     * @param remotePlaylists remote playlists in the display index order
+     * @return merged playlists
+     */
     static List<PlaylistLocalItem> merge(
             final List<PlaylistMetadataEntry> localPlaylists,
             final List<PlaylistRemoteEntity> remotePlaylists) {
-        // The playlists from the database must be in the display index order.
 
-        // Merge localPlaylists and remotePlaylists by display index.
-        // If two items have the same display index, sort them in CASE_INSENSITIVE_ORDER.
+        for (int i = 1; i < localPlaylists.size(); i++) {
+            if (localPlaylists.get(i).getDisplayIndex()
+                    < localPlaylists.get(i - 1).getDisplayIndex()) {
+                throw new IllegalArgumentException(
+                        "localPlaylists is not in the display index order");
+            }
+        }
+
+        for (int i = 1; i < remotePlaylists.size(); i++) {
+            if (remotePlaylists.get(i).getDisplayIndex()
+                    < remotePlaylists.get(i - 1).getDisplayIndex()) {
+                throw new IllegalArgumentException(
+                        "remotePlaylists is not in the display index order");
+            }
+        }
+
         // This algorithm is similar to the merge operation in merge sort.
 
         final List<PlaylistLocalItem> result = new ArrayList<>(
diff --git a/app/src/test/java/org/schabi/newpipe/database/playlist/PlaylistLocalItemTest.java b/app/src/test/java/org/schabi/newpipe/database/playlist/PlaylistLocalItemTest.java
index e5f717144..98f611037 100644
--- a/app/src/test/java/org/schabi/newpipe/database/playlist/PlaylistLocalItemTest.java
+++ b/app/src/test/java/org/schabi/newpipe/database/playlist/PlaylistLocalItemTest.java
@@ -24,16 +24,26 @@ public class PlaylistLocalItemTest {
     public void onlyLocalPlaylists() {
         final List<PlaylistMetadataEntry> localPlaylists = new ArrayList<>();
         final List<PlaylistRemoteEntity> remotePlaylists = new ArrayList<>();
-        localPlaylists.add(new PlaylistMetadataEntry(1, "name1", "", 2, 1));
+        localPlaylists.add(new PlaylistMetadataEntry(1, "name1", "", 0, 1));
         localPlaylists.add(new PlaylistMetadataEntry(2, "name2", "", 1, 1));
-        localPlaylists.add(new PlaylistMetadataEntry(3, "name3", "", 0, 1));
+        localPlaylists.add(new PlaylistMetadataEntry(3, "name3", "", 3, 1));
         final List<PlaylistLocalItem> mergedPlaylists =
                 PlaylistLocalItem.merge(localPlaylists, remotePlaylists);
 
         assertEquals(3, mergedPlaylists.size());
         assertEquals(0, mergedPlaylists.get(0).getDisplayIndex());
         assertEquals(1, mergedPlaylists.get(1).getDisplayIndex());
-        assertEquals(2, mergedPlaylists.get(2).getDisplayIndex());
+        assertEquals(3, mergedPlaylists.get(2).getDisplayIndex());
+    }
+
+    @Test(expected = IllegalArgumentException.class)
+    public void invalidLocalPlaylists() {
+        final List<PlaylistMetadataEntry> localPlaylists = new ArrayList<>();
+        final List<PlaylistRemoteEntity> remotePlaylists = new ArrayList<>();
+        localPlaylists.add(new PlaylistMetadataEntry(1, "name1", "", 2, 1));
+        localPlaylists.add(new PlaylistMetadataEntry(2, "name2", "", 1, 1));
+        localPlaylists.add(new PlaylistMetadataEntry(3, "name3", "", 0, 1));
+        PlaylistLocalItem.merge(localPlaylists, remotePlaylists);
     }
 
     @Test
@@ -41,18 +51,31 @@ public class PlaylistLocalItemTest {
         final List<PlaylistMetadataEntry> localPlaylists = new ArrayList<>();
         final List<PlaylistRemoteEntity> remotePlaylists = new ArrayList<>();
         remotePlaylists.add(new PlaylistRemoteEntity(
-                1, "name1", "url1", "", "", 2, 1L));
+                1, "name1", "url1", "", "", 1, 1L));
         remotePlaylists.add(new PlaylistRemoteEntity(
-                2, "name2", "url2", "", "", 1, 1L));
+                2, "name2", "url2", "", "", 2, 1L));
         remotePlaylists.add(new PlaylistRemoteEntity(
-                3, "name3", "url3", "", "", 0, 1L));
+                3, "name3", "url3", "", "", 4, 1L));
         final List<PlaylistLocalItem> mergedPlaylists =
                 PlaylistLocalItem.merge(localPlaylists, remotePlaylists);
 
         assertEquals(3, mergedPlaylists.size());
-        assertEquals(0, mergedPlaylists.get(0).getDisplayIndex());
-        assertEquals(1, mergedPlaylists.get(1).getDisplayIndex());
-        assertEquals(2, mergedPlaylists.get(2).getDisplayIndex());
+        assertEquals(1, mergedPlaylists.get(0).getDisplayIndex());
+        assertEquals(2, mergedPlaylists.get(1).getDisplayIndex());
+        assertEquals(4, mergedPlaylists.get(2).getDisplayIndex());
+    }
+
+    @Test(expected = IllegalArgumentException.class)
+    public void invalidRemotePlaylists() {
+        final List<PlaylistMetadataEntry> localPlaylists = new ArrayList<>();
+        final List<PlaylistRemoteEntity> remotePlaylists = new ArrayList<>();
+        remotePlaylists.add(new PlaylistRemoteEntity(
+                1, "name1", "url1", "", "", 1, 1L));
+        remotePlaylists.add(new PlaylistRemoteEntity(
+                2, "name2", "url2", "", "", 3, 1L));
+        remotePlaylists.add(new PlaylistRemoteEntity(
+                3, "name3", "url3", "", "", 0, 1L));
+        PlaylistLocalItem.merge(localPlaylists, remotePlaylists);
     }
 
     @Test