diff --git a/tuhi/wacom.py b/tuhi/wacom.py
index 7de5629..0b572bd 100644
--- a/tuhi/wacom.py
+++ b/tuhi/wacom.py
@@ -503,11 +503,11 @@ class WacomProtocolLowLevelComm(GObject.Object):
 
         # logger.debug(f'received {data.opcode:02x} / {data.length:02x} / {b2hex(bytes(data))}')
 
-        if not isinstance(expected_opcode, list):
-            expected_opcode = [expected_opcode]
-
-        if data.opcode not in expected_opcode:
-            raise WacomException(f'unexpected opcode: {data.opcode:02x}')
+        if expected_opcode is not None:
+            if not isinstance(expected_opcode, list):
+                expected_opcode = [expected_opcode]
+            if data.opcode not in expected_opcode:
+                raise WacomException(f'unexpected opcode: {data.opcode:02x}')
 
         return data