diff --git a/interfaces/Stringable.php b/interfaces/Stringable.php
index <HASH>..<HASH> 100644
--- a/interfaces/Stringable.php
+++ b/interfaces/Stringable.php
@@ -28,7 +28,7 @@ interface Stringable
      *
      * @return  string
      */
-    public function toString();
+    public function toString() : string;
 
     /**
      * {@see self::toString()}
[Core] Stringable: hint the return type for toString() while leaving the magic __toString() as-is to avoid conflicts