diff --git a/artiq/runtime.rs/src/session.rs b/artiq/runtime.rs/src/session.rs
index 8ad957a61..8589971a0 100644
--- a/artiq/runtime.rs/src/session.rs
+++ b/artiq/runtime.rs/src/session.rs
@@ -114,7 +114,7 @@ fn host_read(stream: &mut TcpStream) -> io::Result<host::Request> {
     Ok(request)
 }
 
-fn host_write(stream: &mut TcpStream, reply: host::Reply) -> io::Result<()> {
+fn host_write(stream: &mut Write, reply: host::Reply) -> io::Result<()> {
     trace!("comm->host {:?}", reply);
     reply.write_to(stream)
 }
@@ -389,8 +389,9 @@ fn process_kern_message(waiter: Waiter,
                 match stream {
                     None => unexpected!("unexpected RPC in flash kernel"),
                     Some(ref mut stream) => {
-                        try!(host_write(stream, host::Reply::RpcRequest { async: async }));
-                        try!(rpc::send_args(&mut BufWriter::new(stream), service, tag, data));
+                        let writer = &mut BufWriter::new(stream);
+                        try!(host_write(writer, host::Reply::RpcRequest { async: async }));
+                        try!(rpc::send_args(writer, service, tag, data));
                         if !async {
                             session.kernel_state = KernelState::RpcWait
                         }
diff --git a/artiq/test/coredevice/test_rtio.py b/artiq/test/coredevice/test_rtio.py
index f60abd643..061d5fc3c 100644
--- a/artiq/test/coredevice/test_rtio.py
+++ b/artiq/test/coredevice/test_rtio.py
@@ -450,6 +450,8 @@ class RPCTest(ExperimentCase):
                          "timings are dependent on CPU load and network conditions")
     def test_rpc_timing(self):
         self.execute(RPCTiming)
-        self.assertGreater(self.dataset_mgr.get("rpc_time_mean"), 100*ns)
-        self.assertLess(self.dataset_mgr.get("rpc_time_mean"), 15*ms)
-        self.assertLess(self.dataset_mgr.get("rpc_time_stddev"), 2*ms)
+        rpc_time_mean = self.dataset_mgr.get("rpc_time_mean")
+        print(rpc_time_mean)
+        self.assertGreater(rpc_time_mean, 100*ns)
+        self.assertLess(rpc_time_mean, 2*ms)
+        self.assertLess(self.dataset_mgr.get("rpc_time_stddev"), 1*ms)