diff --git a/umodbus/exceptions.py b/umodbus/exceptions.py
index 85a9a56..0f6a1cd 100644
--- a/umodbus/exceptions.py
+++ b/umodbus/exceptions.py
@@ -3,6 +3,16 @@ class ModbusError(Exception):
     pass
 
 
+class UndefinedModbusError(ModbusError):
+    """Catch-all class for non-standard error codes."""
+
+    def __init__(self, error_code):
+        self.error_code = error_code
+
+    def __str__(self):
+        return 'Non-standard Modbus error code %#04x received.' % self.error_code
+
+
 class IllegalFunctionError(ModbusError):
     """ The function code received in the request is not an allowable action for
     the server.
diff --git a/umodbus/functions.py b/umodbus/functions.py
index 4db3cec..ed465d2 100644
--- a/umodbus/functions.py
+++ b/umodbus/functions.py
@@ -74,7 +74,7 @@
 from umodbus import conf, log
 from umodbus.exceptions import (error_code_to_exception_map,
                                 IllegalDataValueError, IllegalFunctionError,
-                                IllegalDataAddressError)
+                                IllegalDataAddressError, UndefinedModbusError)
 from umodbus.utils import memoize, get_function_code_from_request_pdu
 
 # Function related to data access.
@@ -115,7 +115,11 @@ def pdu_to_function_code_or_raise_error(resp_pdu):
 
     if function_code not in function_code_to_function_map.keys():
         error_code = struct.unpack('>B', resp_pdu[1:2])[0]
-        raise error_code_to_exception_map[error_code]
+        try:
+            exception = error_code_to_exception_map[error_code]
+            raise exception
+        except KeyError as e:
+            raise UndefinedModbusError(e.args[0])
 
     return function_code