CST - input current

Hello, I have a stripline circuitry, where I would like to probe the input current. I have created a closed rectangular loop around the center conductor and added H-field monitors, hoping to integrate the curve (Results->Evaluate curve) based on Amphere law and compute "I".

CST MWS returns an error message "unable to determine curve line." Can any of you help? Please let me know. Thanks - Wajih