diff --git a/src/Assert.php b/src/Assert.php
index 7058112..3faecec 100644
--- a/src/Assert.php
+++ b/src/Assert.php
@@ -2102,6 +2102,8 @@ protected static function valueToString($value)
     }
 
     /**
+     * @psalm-pure
+     *
      * @param mixed $value
      *
      * @return string