diff --git a/src/main/java/org/apache/commons/text/StringSubstitutor.java b/src/main/java/org/apache/commons/text/StringSubstitutor.java
index 82e64c9..5057351 100644
--- a/src/main/java/org/apache/commons/text/StringSubstitutor.java
+++ b/src/main/java/org/apache/commons/text/StringSubstitutor.java
@@ -222,7 +222,7 @@
      *
      * @since 1.9
      */
-    public static class Result {
+    public static final class Result {
 
         /** Whether the buffer is altered. */
         public final boolean altered;