diff --git a/src/Assert.php b/src/Assert.php
index 05970a71..175d1f3b 100644
--- a/src/Assert.php
+++ b/src/Assert.php
@@ -1668,6 +1668,9 @@ public static function writable($value, $message = '')
     }
 
     /**
+     * @psalm-pure
+     * @psalm-assert class-string $value
+     *
      * @param mixed  $value
      * @param string $message
      *
diff --git a/tests/static-analysis/assert-classExists.php b/tests/static-analysis/assert-classExists.php
index 1bc81433..88740d7c 100644
--- a/tests/static-analysis/assert-classExists.php
+++ b/tests/static-analysis/assert-classExists.php
@@ -4,9 +4,9 @@
 
 use Webmozart\Assert\Assert;
 
-
 /**
- * @psalm-param class-string $value
+ * @psalm-pure
+ * @psalm-param mixed $value
  *
  * @psalm-return class-string
  */