diff --git a/src/FdfWriter.php b/src/FdfWriter.php
index 9b28665..fe2f6c0 100644
--- a/src/FdfWriter.php
+++ b/src/FdfWriter.php
@@ -85,7 +85,11 @@ public function save(string $filename): int|bool
     public static function escapePdfString(?string $string): string
     {
         // Create UTF-16BE string encode as ASCII hex
-        $utf16Value = mb_convert_encoding($string ?? '', 'UTF-16BE', 'UTF-8');
+        $string ??= '';
+        $utf16Value = mb_convert_encoding($string, 'UTF-16BE', 'UTF-8');
+        if (false === $utf16Value) {
+            return $string;
+        }
 
         return strtr($utf16Value, ['(' => '\(', ')' => '\)', '\\' => '\\\\']);
     }