Index Error- Cannot index type Number (Integer) into type

Im getting Cannot index type Number (Integer) into type (DOCUMENT_DETAILS) - which is a cdt. 

local!indexNum is  just a local

                a!radioButtonField(
                  label: "Radio Buttons",
                  labelPosition: "ABOVE",
                  choiceLabels:rule!SKJ_GetForm(
                    {}

                  )['recordType!{228c141f-e2f8-4a93-bf0d-0dfed8c9c6b4}SKJ Form.fields.{af4b3fdd-a9a3-4ea7-82d9-8f16e323e35f}descEn'],
                  choiceValues:rule!SKJ_GetForm(
                    {}

                  )['recordType!{228c141f-e2f8-4a93-bf0d-0dfed8c9c6b4}SKJ Form.fields.{d69bf265-893b-4707-965c-3512e6e1c9bd}itemCode'],
   
            
                  value:
                  ri!DocDetails[local!indexNum].docForm,
       
                 
           
                  saveInto: {


                    ri!DocDetails[local!indexNum].docForm


                  },
                  required: true,
                  choiceLayout: "COMPACT",
                  validations: {}
                )