How to declare this nested dynamic object in TypeScript?

{
  foo: "bar",
  ["some"]: {
    innerKey: "bar",
  },
  ["other"]: {
    innerKey: "bar",
  },
}

Using this doesn’t work, I get an error because of foo.

interface NestedObject {
  foo: string;
  [key: string]: {
    innerKey: string;
  };
}

Property 'foo' of type 'string' is not assignable to 'string' index type '{ innerKey: string; }'.ts(2411)