diff --git a/library/core/src/main/java/com/google/android/exoplayer2/util/Util.java b/library/core/src/main/java/com/google/android/exoplayer2/util/Util.java
index ceefe758bb..f4637ebde6 100644
--- a/library/core/src/main/java/com/google/android/exoplayer2/util/Util.java
+++ b/library/core/src/main/java/com/google/android/exoplayer2/util/Util.java
@@ -262,7 +262,7 @@ public final class Util {
     try {
       return language == null ? null : new Locale(language).getISO3Language();
     } catch (MissingResourceException e) {
-      return language.toLowerCase();
+      return toLowerInvariant(language);
     }
   }