Thank-you whyisthisme. However, the problem remained after testing the value using an if statement. as suggested. My code is now against the Change Event on the radio-group (rather than the check-box itself), as follows:-Floor.mandatory = (House.rawValue == 1) ?  "disabled" : "error";Floor.presence ...

While testing, I am experiencing an exception behaviour oddity on a runtime mandatory setting of a text field depending on which check-box a user selects...I have two mutually-exclusive check-boxes, House and Appartment, plus a text-field named Floor. If the user selects House, then I disable the ma...