diff --git a/stspin/spin_device.py b/stspin/spin_device.py
index 8b75d09..b3b7b1a 100644
--- a/stspin/spin_device.py
+++ b/stspin/spin_device.py
@@ -91,7 +91,7 @@ def _writeCommand(
         :return: Response bytes as int
         """
         # payload and payload_size must be either both present, or both absent
-        assert((payload is None) != (payload_size is None))
+        assert((payload is None) == (payload_size is None))
 
         response = self._write(command)