diff --git a/src/Illuminate/Http/Concerns/InteractsWithContentTypes.php b/src/Illuminate/Http/Concerns/InteractsWithContentTypes.php
index be760a2619d9..25d6ec1e9986 100644
--- a/src/Illuminate/Http/Concerns/InteractsWithContentTypes.php
+++ b/src/Illuminate/Http/Concerns/InteractsWithContentTypes.php
@@ -31,7 +31,7 @@ public static function matchesType($actual, $type)
      */
     public function isJson()
     {
-        return Str::contains($this->header('CONTENT_TYPE'), ['/json', '+json']);
+        return Str::contains($this->header('CONTENT_TYPE') ?? '', ['/json', '+json']);
     }
 
     /**