¿Por qué handleChange('name', 42) y handleChange('unknown', 'value') dan error?

type FormFields = {
  name: string;
  email: string;
  age: number;
};

const useForm = <T extends Record<string, unknown>>(initial: T) => {
  const [values, setValues] = useState<T>(initial);

  const handleChange = <K extends keyof T>(
    field: K,
    value: T[K]
  ) => {
    setValues(prev => ({ ...prev, [field]: value }));
  };

  return { values, handleChange };
};

const { values, handleChange } = useForm<FormFields>({
  name: "",
  email: "",
  age: 0,
});

handleChange("name", "Alice");     // OK
handleChange("age", 25);           // OK
handleChange("name", 42);          // Error!
handleChange("unknown", "value");  // Error!