Problem with condition for disable subform

Problem with condition for disable subform

Good day,

I have a subform in which I want a radio or checkbox type field by selecting an option from the list in the same place two fields subformularia DISABLED. and trying to do it this way.
if (sfrmSubActividades.chkSubnivel3 == "Si")
{
disable sfrmSubActividades.txtValorSubcUE;
disable sfrmSubActividades.txtValorSubcGN;
}
  InvalidTaskInSubForm get error, can in some way?