diff --git a/scrapy/downloadermiddlewares/uriuserinfo.py b/scrapy/downloadermiddlewares/uriuserinfo.py
index 6deb5b0eb..ba7ee0f1d 100644
--- a/scrapy/downloadermiddlewares/uriuserinfo.py
+++ b/scrapy/downloadermiddlewares/uriuserinfo.py
@@ -22,14 +22,14 @@ class UriUserInfoMiddleware:
     def process_request(self, request, spider):
         parsed_url = urlparse_cached(request)
         if parsed_url.username is None and parsed_url.password is None:
-            return
+            return None
 
         if parsed_url.scheme.startswith("http"):
             username_field, password_field = "http_user", "http_pass"
         elif parsed_url.scheme.startswith("ftp"):
             username_field, password_field = "ftp_user", "ftp_password"
         else:
-            return
+            return None
 
         for key, value in (
             (username_field, parsed_url.username),