diff --git a/app.py b/app.py
index e7ede6a..cc85ce4 100644
--- a/app.py
+++ b/app.py
@@ -80,7 +80,10 @@
 
 def update(roc, prom, old):
     new = roc.update_devices(old)
-    prom.update_devices(new)
+    if new is not None:
+        new = prom.update_devices(new)
+    else:
+        new = old
     return new
 
 def probe(devices):