emberjs glimmer object set() with variable property name

I have a component in Ember 1.15 where I am trying to do something like

import { action, set } from '@ember/object';

@action
someMethod() {
  const value = ... // something random
  let propertyName = ... // some variable string
  set(this, propertyName, value);
}

It seems to be working fine in the browser but typescript is flagging the set line as an error (specifically the propertyName argument). So if it works, why doesn't typescript like it?

This also seems to be happening with get() where it doesn't like variable propertyNames like get(this, propertyName).