diff --git a/library/core/src/main/java/com/google/android/exoplayer2/upstream/cache/SimpleCache.java b/library/core/src/main/java/com/google/android/exoplayer2/upstream/cache/SimpleCache.java
index 98c1fc97f7..d1b26f884c 100644
--- a/library/core/src/main/java/com/google/android/exoplayer2/upstream/cache/SimpleCache.java
+++ b/library/core/src/main/java/com/google/android/exoplayer2/upstream/cache/SimpleCache.java
@@ -306,14 +306,16 @@ public final class SimpleCache implements Cache {
   }
 
   @Override
-  public void applyContentMetadataMutations(String key, ContentMetadataMutations mutations)
-      throws CacheException {
+  public synchronized void applyContentMetadataMutations(
+      String key, ContentMetadataMutations mutations) throws CacheException {
+    Assertions.checkState(!released);
     index.applyContentMetadataMutations(key, mutations);
     index.store();
   }
 
   @Override
-  public ContentMetadata getContentMetadata(String key) {
+  public synchronized ContentMetadata getContentMetadata(String key) {
+    Assertions.checkState(!released);
     return index.getContentMetadata(key);
   }