Apple Watch - Err displayed as value for an item

I try to calcute a value for energy consumption with the following rule:
rule “Verbrauch_calc”
Item KacoW changed or
Item Akku_PowerOut changed or
Item T_Power changed
Verbrauch.postUpdate((KacoW.state as Number) + (Akku_PowerOut.state as Number) + (T_Power.state as Number))

The value of Item Verbrauch got displayed correctly in my sitemap when I look it up in the browser. Trying the same on iOS and Apple Watch, I only see the value ‘err’. Any idea why this goes wrong ?

solved: item definition has to be number, not string. Don’t know why the behgaviour is different on Browser, iOS and WatchOS.