diff --git a/src/exception.toit b/src/exception.toit
index a6319bb..4882f4f 100644
--- a/src/exception.toit
+++ b/src/exception.toit
@@ -32,7 +32,11 @@ class ModbusException:
     transaction-id = frame.transaction-id
     data = frame
 
+  constructor.noise --.message --.data:
+    code = CORRUPTED
+    transaction-id = Frame.NO-TRANSACTION-ID
+
   constructor.other .code --.transaction-id --.message --.data:
 
   stringify -> string:
-    return "Invalid frame $message"
+    return "Invalid frame: $message"
diff --git a/src/rs485.toit b/src/rs485.toit
index 3ec1da8..fd58cae 100644
--- a/src/rs485.toit
+++ b/src/rs485.toit
@@ -84,6 +84,12 @@ class RtuFramer implements Framer:
             closed = true
     last-activity-us_ = Time.monotonic-us
 
+    if data.size < 4:
+      exception := ModbusException.noise
+          --message="too short"
+          --data=data
+      throw exception
+
     unit-id := data[0]
     function-code := data[1]
     frame-data := data[2..data.size - 2]
diff --git a/tests/README.md b/tests/README.md
index 64ce9f5..fc2512b 100644
--- a/tests/README.md
+++ b/tests/README.md
@@ -10,6 +10,10 @@ The safe choice is to consider them BSD as well.
 
 ## Installation
 
+Use `requirements.txt` to install the Python dependencies.
+
+For reference:
+
 pymodbus=3.0.0.dev4 requires the 'imp' module, which was removed with Python 3.12.
 
 Install with
@@ -18,7 +22,8 @@ Install with
 pip install -U 'pymodbus==3.0.0.dev4' serial
 ```
 
-Note: we can't currently open the serial port in Toit-desktop. The following instructions are thus not yet relevant.
+Note: we currently don't test the UART version. The following
+instructions are thus not yet relevant.
 
 To test the serial rtu client, create a pipe as follows:
 ``` shell
diff --git a/tests/rtu_bad_transport_test_no_external.toit b/tests/rtu_bad_transport_test_no_external.toit
new file mode 100644
index 0000000..72bdb8b
--- /dev/null
+++ b/tests/rtu_bad_transport_test_no_external.toit
@@ -0,0 +1,108 @@
+// Copyright (C) 2022 Toitware ApS.
+// Use of this source code is governed by a Zero-Clause BSD license that can
+// be found in the TESTS_LICENSE file.
+
+import expect show *
+import io
+import log
+import modbus
+import modbus.rs485 as modbus
+import modbus.tcp as modbus
+import modbus.exception as modbus
+import modbus.framer as modbus
+import net
+
+import .common as common
+import .test-server
+
+class BadFramer implements modbus.Framer:
+  wrapped_/modbus.Framer
+  bad-next-read-bytes_/ByteArray? := null
+  bad-next-read-frame_/modbus.Frame? := null
+  eat-frame/bool := false
+  last-response/modbus.Frame? := null
+
+  constructor .wrapped_:
+
+  read reader/io.Reader -> modbus.Frame:
+    if bad-next-read-bytes_:
+      data := bad-next-read-bytes_
+      bad-next-read-bytes_ = null
+      return wrapped_.read (io.Reader data)
+    if bad-next-read-frame_:
+      frame := bad-next-read-frame_
+      bad-next-read-frame_ = null
+      return frame
+    if eat-frame:
+      intercepted := wrapped_.read reader
+      print_ intercepted
+      sleep --ms=100000
+    result := wrapped_.read reader
+    // We check for the "eat-frame" after the 'read' as the
+    // bus is reading the frames asynchronously before a request was sent.
+    // That means that we enter the wrapped reader's read method before
+    // the test has set the 'eat-frame' variable.
+    if eat-frame:
+      eat-frame = false
+      return read reader
+    last-response = result
+    return result
+
+  write frame/modbus.Frame writer/io.Writer:
+    wrapped_.write frame writer
+
+main args:
+  server-logger := (log.default.with-level log.INFO-LEVEL).with-name "server"
+  with-test-server --logger=server-logger --mode="tcp_rtu":
+    test it
+
+test port/int:
+  net := net.open
+
+  socket := net.tcp-connect "localhost" port
+
+  original-framer := modbus.RtuFramer --baud-rate=9600
+  framer := BadFramer original-framer
+  transport := modbus.TcpTransport socket --framer=framer
+  bus := modbus.Modbus transport
+
+  station := bus.station 1
+  holding := station.holding-registers
+  holding.write-single --address=50 42
+  holding.write-single --address=51 43
+
+  // Check that a spurious read does not cause an error.
+  // Note that the bus already started reading. So one frame will
+  // make it through without errors, but the next one will have the
+  // garbage.
+  // The output of the test should show a
+  //    "WARN: exception: Invalid frame: too short"
+  // We simply do two reads.
+  framer.bad-next-read-bytes_ = #[0x00, 0x01]
+  data := holding.read-single --address=50
+  expect-equals 42 data
+  data = holding.read-single --address=51
+  expect-equals 43 data
+
+  // Check that the bus recovers when frames are lost.
+  framer.eat-frame = true
+  expect-throw DEADLINE-EXCEEDED-ERROR:
+    holding.read-single --address=50
+  framer.eat-frame = false
+
+  // Check that the bus recovers when frames are lost.
+  data = holding.read-single --address=50
+  expect-equals 42 data
+
+  // Send a valid response when no one is expecting it.
+  framer.bad-next-read-frame_ = framer.last-response
+
+  // When the next frame is set, the framer is already reading from the
+  // uart. So we need to do one normal read which consumes the actual UART
+  // data before the bad frame is used.
+  // The output of the test should show a
+  //   "WARN: unpaired response or multiple responses"
+  data = holding.read-single --address=51
+  expect-equals 43 data
+
+  bus.close