diff --git a/src/UnresolvedValueException.php b/src/UnresolvedValueException.php
index 0e1d38b..c1de7f9 100644
--- a/src/UnresolvedValueException.php
+++ b/src/UnresolvedValueException.php
@@ -53,7 +53,7 @@ private static function getParameterName(\ReflectionParameter $parameter) : stri
         return $parameterType . '$' . $parameter->getName();
     }
 
-    private static function getReflectionTypeName(\ReflectionType $reflectionType)
+    private static function getReflectionTypeName(\ReflectionType $reflectionType) : string
     {
         if ($reflectionType instanceof \ReflectionNamedType) {
             return $reflectionType->getName();