diff --git a/src/Map/src/Renderer/Renderers.php b/src/Map/src/Renderer/Renderers.php
index ea7fae8eed1..ea6262ab568 100644
--- a/src/Map/src/Renderer/Renderers.php
+++ b/src/Map/src/Renderer/Renderers.php
@@ -57,7 +57,7 @@ public function renderMap(Map $map, array $attributes = []): string
         return $renderer->renderMap($map, $attributes);
     }
 
-    public function __toString()
+    public function __toString(): string
     {
         return implode(', ', array_keys($this->renderers));
     }