diff --git a/src/java.base/share/classes/jdk/internal/lang/CaseFolding.java.template b/src/java.base/share/classes/jdk/internal/lang/CaseFolding.java.template
index 24a183c8da0..24f48151f21 100644
--- a/src/java.base/share/classes/jdk/internal/lang/CaseFolding.java.template
+++ b/src/java.base/share/classes/jdk/internal/lang/CaseFolding.java.template
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2025, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2025, 2026, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -188,6 +188,12 @@ public final class CaseFolding {
     }
 
     private static long getDefined(int cp) {
+        // Exclude code point U+0000, which is guaranteed to have no
+        // case-folding mapping.
+        if (cp == 0) {
+            return -1;
+        }
+
         var hashes = CASE_FOLDING_HASHES;
         var length = CASE_FOLDING_CPS.length;  // hashed based on total defined.
         var hash = cp % length;