Unable to invoke RPC using NETCONF on mounted device

When a RPC call was made to a NETCONF monted device we see the following error.

curl -u admin:admin -X POST -H "Accept: application/json" -H "Content-type: application/json" -d @vrouter.interface.rpc.netconf.json http://localhost:8181/restconf/operations/opendaylight-inventory:nodes/node/110-41R3/yang-ext:mount/vyatta-op-v1:interface | python -m json.tool

{
"errors": {
"error": [

{ "error-message": "URI has bad format. Possible reasons:\n 1. \"vyatta-op-v1:interface\" was not found in parent data node.\n 2. \"vyatta-op-v1:interface\" is behind mount point. Then it should be in format \"/yang-ext:mount/vyatta-op-v1:interface\".", "error-tag": "invalid-value", "error-type": "protocol" }

]
}
}

The yang model in question is resolved properly and URLs can be seen in API doc explorer. We can make other calls on config and operational trees but not RPCs.