diff --git a/src/Message.php b/src/Message.php
index 9b74b8d4..6e6c3e51 100644
--- a/src/Message.php
+++ b/src/Message.php
@@ -33,7 +33,7 @@ public static function toString(MessageInterface $message): string
         }
 
         foreach ($message->getHeaders() as $name => $values) {
-            if (strtolower($name) === 'set-cookie') {
+            if (is_string($name) && strtolower($name) === 'set-cookie') {
                 foreach ($values as $value) {
                     $msg .= "\r\n{$name}: ".$value;
                 }
diff --git a/src/MessageTrait.php b/src/MessageTrait.php
index a85d3ab2..e05ebea8 100644
--- a/src/MessageTrait.php
+++ b/src/MessageTrait.php
@@ -12,10 +12,10 @@
  */
 trait MessageTrait
 {
-    /** @var array<string, string[]> Map of all registered headers, as original name => array of values */
+    /** @var string[][] Map of all registered headers, as original name => array of values */
     private $headers = [];
 
-    /** @var array<string, string> Map of lowercase header name => original name at registration */
+    /** @var string[] Map of lowercase header name => original name at registration */
     private $headerNames = [];
 
     /** @var string */