React: checker is not a function

In my case I got this when I used the shape function with a complex object.
The solution was to go from:

outerObject: shape({
  firstInnerObject: {
    a: string,
    b: string,
  },
  secondInnerObject: {
    a: string,
    b: number,
  },
}),

To:

outerObject: shape({
  firstInnerObejct: shape({
    a: string,
    b: string,
  }),
  secondInnerObject: shape({
    a: string,
    b: number,
  }),
}),

Very trivial, I know, yet this might be the solution for someone equally inexperienced as I am. 😉