Design Patterns: Generic Is Alive or just using the Expire binding might be a better overall approach.
You should use UNDEF as the state instead of NULL. NULL means not initialized. UNDEF means we don’t know what state the device is in. In this case, UNDEF is more appropriate.
I don’t see anything obviously wrong that would generate that error. Are you sure verisure_lastupdate doesn’t have a NULL or UNDEF state? I could see that error occurring if that were the case.