Web virtual-server does not handle undefining default-web-module properly

Execute the following commands from the CLI:

/subsystem=web/virtual-server=default-host:write-attribute(name=enable-welcome-root, value=false)
/subsystem=web/virtual-server=default-host:write-attribute(name=default-web-module, value=mywar)
/subsystem=web/virtual-server=default-host:undefine-attribute(name=default-web-module)
/subsystem=web/virtual-server=default-host:write-attribute(name=enable-welcome-root, value=true)

The last command will fail with this error:
{
"outcome" => "failed",
"failure-description" => "JBAS018011: The welcome root can not be enabled on a host that has the default web module",
"rolled-back" => true,
"response-headers" =>

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

If I run read-resource, I see this:
{
"outcome" => "success",
"result" => { "access-log" => undefined, "alias" => [ "localhost", "example.com" ], "default-web-module" => "undefined", "enable-welcome-root" => false, "name" => "default-host", "rewrite" => undefined, "sso" => undefined },
"response-headers" => {"process-state" => "reload-required"}

}

It has put in a string of "undefined" instead of actually undefining the attribute.