diff --git a/check_zigbee_battery.sh b/check_zigbee_battery.sh
index 0c4e362..66ed3ec 100755
--- a/check_zigbee_battery.sh
+++ b/check_zigbee_battery.sh
@@ -30,11 +30,17 @@ ZigBeeDeviceCritical=$(expr $ZigBeeDeviceCritical + 0)
 ZigBeeDeviceBattery=$(curl -S -m 5 http://172.30.80.246:8087/getPlainValue/zigbee.0.${ZigBeeDeviceID}.battery 2>/dev/null)
 
 if [ $? -ne 0 ] ; then
-	echo "ioBroker-API not available or wrong Zigbee-ID or no battery present."
+	echo "ioBroker-API not available or wrong Zigbee-ID or no battery present"
 	beenden 3
 fi
 
-ZigBeeDeviceBattery=$(expr $ZigBeeDeviceBattery + 0)
+ZigBeeDeviceBattery=$(expr $ZigBeeDeviceBattery + 0 2>/dev/null)
+
+if [ $? -ne 0 ] ; then
+        echo "Unknown: Got unknown value from ioBroker"
+        beenden 3
+fi
+
 
 if [ $ZigBeeDeviceBattery -le $ZigBeeDeviceCritical ] ; then
 	echo "Critical: Battery is at ${ZigBeeDeviceBattery}%, which is below Critical-Value (${ZigBeeDeviceCritical}%) | battery_percent=${ZigBeeDeviceBattery}"