diff --git a/python/qemu/qmp.py b/python/qemu/qmp.py
index a634c4e26c..e64b6b5faa 100644
--- a/python/qemu/qmp.py
+++ b/python/qemu/qmp.py
@@ -94,6 +94,7 @@ class QEMUMonitorProtocol:
         raise QMPCapabilitiesError
 
     def __json_read(self, only_event=False):
+        assert self.__sockfile is not None
         while True:
             data = self.__sockfile.readline()
             if not data: