diff --git a/src/Helper/StringStream.php b/src/Helper/StringStream.php
index 22255b8a..e57a2fe9 100755
--- a/src/Helper/StringStream.php
+++ b/src/Helper/StringStream.php
@@ -32,7 +32,7 @@ public function __construct(
         rewind($this->stream);
     }
 
-    public function __toString()
+    public function __toString(): string
     {
         $stream = $this->stream;
         if ($stream === null) {