Improved error handling for calculated values
Improved error handling for calculated values

mrkeng/alerta,guardian/alerta,skob/alerta,skob/alerta,skob/alerta,guardian/alerta,skob/alerta,0312birdzhang/alerta,mrkeng/alerta,mrkeng/alerta,guardian/alerta,0312birdzhang/alerta,0312birdzhang/alerta,guardian/alerta,mrkeng/alerta