Error when adding configuration child to configadmin subsystem

I get the following error when I try to add a configuration child to subsystem=configadmin:

[standalone@localhost:9999 /] cd subsystem=configadmin
[standalone@localhost:9999 subsystem=configadmin] ./configuration=default:add()
{
"outcome" => "failed",
"failure-description" => "JBAS014749: Operation handler failed: null",
"rolled-back" => true,
"response-headers" =>

{"process-state" => "reload-required"}

}