diff --git a/src/Service/Builder.php b/src/Service/Builder.php
index c96878d..0092b7b 100644
--- a/src/Service/Builder.php
+++ b/src/Service/Builder.php
@@ -133,7 +133,7 @@ class Builder
         return sprintf('(%s)', collect($value)->implode($separator));
     }
 
-    private function glue()
+    private function glue(): ?string
     {
         return $this->expanded ? PHP_EOL : null;
     }