Why immer.js doesn't allow setting dynamic properties on draft?

//I want my action to dispatch payload like
// {type:'update',payload:{'current.contact.mobile':'XXXXXXXXX'}}
//In reducer dynamically select the segment of state update needs to be applied to 
//Below code doesn't work as expected though, draft always remains at same level
draft = dA.key.split('.').reduce((draft, k) => {
  return draft[k]
}, draft);

//Or an ideal syntax may look like below line
draft['current.contact.mobile'] = dA.value;


//Code that works
draft['current']['contact']['mobile'] = dA.value;

I want my action to dispatch payload like {type:'update',payload:{'current.contact.mobile':'XXXXXXXXX'}} And in reducer dynamically select the segment of state that needs to be updated. Is there something fundamentally wrong in doing this, I believe this could make life easier. Is there something that can done to achieve this ?

from Why immer.js doesn't allow setting dynamic properties on draft?