diff --git a/src/Headers.php b/src/Headers.php
index c024886..038e792 100644
--- a/src/Headers.php
+++ b/src/Headers.php
@@ -178,7 +178,7 @@ protected function parseAuthorizationHeader(array $headers): array
     {
         $hasAuthorizationHeader = false;
         foreach ($headers as $name => $value) {
-            if (strtolower($name) === 'authorization') {
+            if (strtolower((string) $name) === 'authorization') {
                 $hasAuthorizationHeader = true;
                 break;
             }