diff --git a/qubes/vm/mix/net.py b/qubes/vm/mix/net.py
index 85d06dd2..0562d131 100644
--- a/qubes/vm/mix/net.py
+++ b/qubes/vm/mix/net.py
@@ -412,7 +412,8 @@ class NetVMMixin(qubes.events.Emitter):
                     'Cannot dynamically attach to stopped NetVM: {!r}'.format(
                         newvalue))
 
-        if oldvalue is not None:
+        # don't check oldvalue, because it's missing if it was default
+        if self.netvm is not None:
             if self.is_running():
                 self.detach_network()