diff --git a/okhttp/src/main/java/com/squareup/okhttp/Headers.java b/okhttp/src/main/java/com/squareup/okhttp/Headers.java
index f397616d6..c1bf0c504 100644
--- a/okhttp/src/main/java/com/squareup/okhttp/Headers.java
+++ b/okhttp/src/main/java/com/squareup/okhttp/Headers.java
@@ -168,7 +168,7 @@ public final class Headers {
     return new Headers(namesAndValues);
   }
 
-  public static class Builder {
+  public static final class Builder {
     private final List<String> namesAndValues = new ArrayList<>(20);
 
     /** Add an header line containing a field name, a literal colon, and a value. */