hierarchy between two 'grouping' elements throws NullPoinerException during generating code

In yangtools (version:0.6.2-SNAPSHOT), there is an error:
based on RFC6020, hierarchy between two 'grouping' elements is allowed. But Yangtools throw NullPointerException. See stacktrace in attachment as a reaction to the yang file content:
module my {
namespace "urn:error";
prefix my;
revision 2011-12-07

{ reference "me"; }

grouping openflow-port-resource-grouping {
grouping openflow-port-base-tunnel-grouping {
choice local-endpoint-address {
leaf name

{ type string; }

}
}
}
container capable-switch {
container resources {
list port

{ key "resource-id"; unique "resource-id"; uses openflow-port-resource-grouping; }

}
}
}