diff --git a/lib/googleauth/service_account.rb b/lib/googleauth/service_account.rb
index f7be414..84a4bfe 100644
--- a/lib/googleauth/service_account.rb
+++ b/lib/googleauth/service_account.rb
@@ -188,15 +188,15 @@ def principal
 
       def apply_self_signed_jwt! a_hash
         # Use the ServiceAccountJwtHeaderCredentials using the same cred values
-        cred_json = {
-          private_key: @signing_key.to_s,
-          client_email: @issuer,
-          project_id: @project_id,
-          quota_project_id: @quota_project_id
-        }
-        key_io = StringIO.new MultiJson.dump(cred_json)
-        alt = ServiceAccountJwtHeaderCredentials.make_creds json_key_io: key_io, scope: scope
-        alt.logger = logger
+        alt = ServiceAccountJwtHeaderCredentials.new(
+          private_key:      @signing_key.to_s,
+          issuer:           @issuer,
+          project_id:       @project_id,
+          quota_project_id: @quota_project_id,
+          universe_domain:  universe_domain,
+          scope:            scope,
+          logger:           logger
+        )
         alt.apply! a_hash
       end