diff --git a/google/cloud/storage/client.py b/google/cloud/storage/client.py
index 4927aecf9..4b23de04e 100644
--- a/google/cloud/storage/client.py
+++ b/google/cloud/storage/client.py
@@ -928,7 +928,7 @@ def generate_signed_post_policy_v4(
         credentials=None,
         virtual_hosted_style=False,
         bucket_bound_hostname=None,
-        scheme=None,
+        scheme="http",
         service_account_email=None,
         access_token=None,
     ):