diff --git a/src/testdir/test_channel.py b/src/testdir/test_channel.py
index 039a9de..311fbc0 100644
--- a/src/testdir/test_channel.py
+++ b/src/testdir/test_channel.py
@@ -130,6 +130,16 @@
                     elif decoded[1] == 'eval-result':
                         # Send back the last received eval result.
                         response = last_eval
+                    elif decoded[1] == 'call me':
+                        cmd = '[0,"we called you"]'
+                        print("sending: {}".format(cmd))
+                        self.request.sendall(cmd.encode('utf-8'))
+                        response = "ok"
+                    elif decoded[1] == 'call me again':
+                        cmd = '[0,"we did call you"]'
+                        print("sending: {}".format(cmd))
+                        self.request.sendall(cmd.encode('utf-8'))
+                        response = ""
                     elif decoded[1] == '!quit!':
                         # we're done
                         self.server.shutdown()
@@ -140,9 +150,12 @@
                     else:
                         response = "what?"
 
-                    encoded = json.dumps([decoded[0], response])
-                    print("sending: {}".format(encoded))
-                    self.request.sendall(encoded.encode('utf-8'))
+                    if response == "":
+                        print("no response")
+                    else:
+                        encoded = json.dumps([decoded[0], response])
+                        print("sending: {}".format(encoded))
+                        self.request.sendall(encoded.encode('utf-8'))
 
                 # Negative numbers are used for "eval" responses.
                 elif decoded[0] < 0:
diff --git a/src/testdir/test_channel.vim b/src/testdir/test_channel.vim
index 8193703..d77778a 100644
--- a/src/testdir/test_channel.vim
+++ b/src/testdir/test_channel.vim
@@ -204,6 +204,35 @@
   call s:run_server('s:server_crash')
 endfunc
 
+let s:reply = ""
+func s:Handler(chan, msg)
+  let s:reply = a:msg
+endfunc
+
+func s:channel_handler(port)
+  let chopt = copy(s:chopt)
+  let chopt['callback'] = 's:Handler'
+  let handle = ch_open('localhost:' . a:port, chopt)
+  if handle < 0
+    call assert_false(1, "Can't open channel")
+    return
+  endif
+
+  " Test that it works while waiting on a numbered message.
+  call assert_equal('ok', ch_sendexpr(handle, 'call me'))
+  sleep 10m
+  call assert_equal('we called you', s:reply)
+
+  " Test that it works while not waiting on a numbered message.
+  call ch_sendexpr(handle, 'call me again', 0)
+  sleep 10m
+  call assert_equal('we did call you', s:reply)
+endfunc
+
+func Test_channel_handler()
+  call s:run_server('s:channel_handler')
+endfunc
+
 " Test that trying to connect to a non-existing port fails quickly.
 func Test_connect_waittime()
   let start = reltime()