diff --git a/lib/googleauth/impersonated_service_account.rb b/lib/googleauth/impersonated_service_account.rb
index b659c4f..6c946be 100644
--- a/lib/googleauth/impersonated_service_account.rb
+++ b/lib/googleauth/impersonated_service_account.rb
@@ -296,9 +296,7 @@ def fetch_access_token! _options = {}
       # @private
       # @return [Hash] The authorization header with the source credentials' token
       def prepare_auth_header
-        auth_header = {}
-        @source_credentials.updater_proc.call auth_header
-        auth_header
+        @source_credentials.updater_proc.call {}
       end
 
       # Makes the HTTP request to the impersonation endpoint.