diff --git a/src/pv_controller/MeterPublish.py b/src/pv_controller/MeterPublish.py
index 0d80243..850bc0b 100644
--- a/src/pv_controller/MeterPublish.py
+++ b/src/pv_controller/MeterPublish.py
@@ -9,7 +9,7 @@ def floatAdaptor(i):
     return float(f"{i:0.2f}") if i else 0.0
 
 def onOffAdaptor(i):
-    return i[0]
+    return i[0] if i else '-1'
 
 
 REGISTERS = [