diff --git a/system/ThirdParty/Escaper/Escaper.php b/system/ThirdParty/Escaper/Escaper.php
index 1e5bc7f9fad9..4fce36bd0c6c 100644
--- a/system/ThirdParty/Escaper/Escaper.php
+++ b/system/ThirdParty/Escaper/Escaper.php
@@ -4,11 +4,13 @@
 
 namespace Laminas\Escaper;
 
+use function assert;
 use function bin2hex;
 use function ctype_digit;
 use function hexdec;
 use function htmlspecialchars;
 use function in_array;
+use function is_string;
 use function mb_convert_encoding;
 use function ord;
 use function preg_match;
@@ -207,6 +209,8 @@ public function escapeHtmlAttr(string $string)
         }
 
         $result = preg_replace_callback('/[^a-z0-9,\.\-_]/iSu', $this->htmlAttrMatcher, $string);
+        assert(is_string($result));
+
         return $this->fromUtf8($result);
     }
 
@@ -229,6 +233,8 @@ public function escapeJs(string $string)
         }
 
         $result = preg_replace_callback('/[^a-z0-9,\._]/iSu', $this->jsMatcher, $string);
+        assert(is_string($result));
+
         return $this->fromUtf8($result);
     }
 
@@ -258,6 +264,8 @@ public function escapeCss(string $string)
         }
 
         $result = preg_replace_callback('/[^a-z0-9]/iSu', $this->cssMatcher, $string);
+        assert(is_string($result));
+
         return $this->fromUtf8($result);
     }