Type 'number' is not assignable to type 'never' when assigning to object's property

TypeScript compiler gives me Type 'number' is not assignable to type 'never' error for the data[key] = 2; assignment in the code snippet below even though I explicitly check the type before the assignment (see on TypeScript Playground).

What is causing the error and what would be a proper way to manipulate all number properties of an object?

const data = {
  a: 1,
  b: 'b',
};

(Object.keys(data) as Array<keyof typeof data>).forEach((key) => {
  if (typeof data[key] === 'number') data[key] = 2;
});