diff --git a/umodbus/client/serial/rtu.py b/umodbus/client/serial/rtu.py
index 4c01580..4ffe485 100644
--- a/umodbus/client/serial/rtu.py
+++ b/umodbus/client/serial/rtu.py
@@ -45,6 +45,7 @@
 import struct
 
 from umodbus.client.serial.redundancy_check import get_crc, validate_crc
+from umodbus.client.serial.redundancy_check import CRCError
 from umodbus.functions import (create_function_from_response_pdu,
                                expected_response_pdu_size_from_request_pdu,
                                pdu_to_function_code_or_raise_error, ReadCoils,
@@ -199,7 +200,13 @@ def raise_for_exception_adu(resp_adu):
     :raises ModbusError: When a response contains an error code.
     """
     resp_pdu = resp_adu[1:-2]
-    pdu_to_function_code_or_raise_error(resp_pdu)
+    try:
+        validate_crc(resp_adu)
+        pdu_to_function_code_or_raise_error(resp_pdu)
+    except CRCError:
+        # The response cannot be trusted at all, so ignore any
+        # possibly invalid function code.
+        pass
 
 
 def send_message(adu, serial_port):
diff --git a/umodbus/functions.py b/umodbus/functions.py
index 4db3cec..f46b1a6 100644
--- a/umodbus/functions.py
+++ b/umodbus/functions.py
@@ -104,11 +104,10 @@
 
 
 def pdu_to_function_code_or_raise_error(resp_pdu):
-    """ Parse response PDU and return of :class:`ModbusFunction` or
-    raise error.
+    """Parse response PDU and return function code or raise error.
 
     :param resp_pdu: PDU of response.
-    :return: Subclass of :class:`ModbusFunction` matching the response.
+    :return: Function code contained in the response.
     :raises ModbusError: When response contains error code.
     """
     function_code = struct.unpack('>B', resp_pdu[0:1])[0]