custom validator and scope

gistfile1.gradle

//the follow resides in Config.groovy

//custom validator that checks if a linked field is identical
	confirmField(validator: { val, obj ->
		String linkedField = propertyName + "Confirm" //the error line
		if(obj.properties[linkedField] != val) {
			return ['confirmField.mismatch']
		}
		return true
	})


//gives error:
No signature of method: groovy.util.ConfigObject.plus() is applicable for argument types: (java.lang.String) values: [Confirm]


//How to reference the propertyName like in http://www.grails.org/doc/latest/ref/Constraints/validator.html ?