Always get ConflictingModificationException when writing

I'm trying to write to the operational store from an RPC method. The problem is that even though there's only one write happening anywhere and no deletes, nonetheless I always get back:

org.opendaylight.controller.sal.restconf.impl.RestconfDocumentedException: errors: [error-type: application, error-tag: operation-failed, error-message: The operation encountered an unexpected error while executing.error-info: org.opendaylight.yangtools.yang.data.api.schema.tree.ConflictingModificationAppliedException: Node was deleted by other transaction.

A slightly simplified version of the code is below. This was working with the old API.

Function<RpcResult<TransactionStatus>, RpcResult<Void>> futureTrans =
new Function<RpcResult<TransactionStatus>,RpcResult<Void>>() {
@Override
public RpcResult<Void> apply(RpcResult<TransactionStatus> input)

{ return Rpcs.<Void>getRpcResult(input.isSuccessful(), input.getErrors()); }

};

public Future<RpcResult<Void>>
registerEndpoint(RegisterEndpointInput input) {
WriteTransaction t = dataProvider.newWriteOnlyTransaction();

Endpoint ep = buildEndpoint(input)
.setTimestamp(timestamp)
.build();

EndpointKey key =
new EndpointKey(ep.getL2Context(), ep.getMacAddress());
InstanceIdentifier<Endpoint> iid =
InstanceIdentifier.builder(Endpoints.class)
.child(Endpoint.class, key)
.build();

t.put(LogicalDatastoreType.OPERATIONAL, iid, ep);

ListenableFuture<RpcResult<TransactionStatus>> r = t.commit();
return Futures.transform(r, futureTrans, executor);
}