diff --git a/pymodbus/pdu.py b/pymodbus/pdu.py
index d73b841cd..64c48b1e1 100644
--- a/pymodbus/pdu.py
+++ b/pymodbus/pdu.py
@@ -164,6 +164,7 @@ class ModbusExceptions:  # pylint: disable=too-few-public-methods
     SlaveFailure = 0x04
     Acknowledge = 0x05
     SlaveBusy = 0x06
+    NegativeAcknowledge = 0x07
     MemoryParityError = 0x08
     GatewayPathUnavailable = 0x0A
     GatewayNoResponse = 0x0B