diff --git a/Demo.php b/Demo.php
index b518a6b..dba8c7e 100644
--- a/Demo.php
+++ b/Demo.php
@@ -12,5 +12,5 @@
 echo khmerDate('2024-01-01')->format();
 
 print_r(khmerDate('2024-01-01')::khNewYear());
-
+// or
 print_r(KhmerNewYearDate(2024));
diff --git a/src/ToKhmerDate.php b/src/ToKhmerDate.php
index 360e549..8976cfa 100644
--- a/src/ToKhmerDate.php
+++ b/src/ToKhmerDate.php
@@ -87,6 +87,11 @@ public static function format(string $format = ''): string
                 },
             ];
             $result = preg_replace_callback('/[a-zA-Z]/', function ($matches) use ($formatRules) {
+
+                // fix  Undefined array key 
+                if (!isset($formatRules[$matches[0]])) {
+                    return $matches[0];
+                }
                 return $formatRules[$matches[0]]();
             }, $format);
             return Constant::postformat($result);