typescript not recognising type for the tupple returned

Question

Below code throws an error when the tuppleFunc is used in the error field for the payload.

interface FormItemPayload {
  value: string;
  error: boolean;
  errorMsg: string;
}

const tuppleFunc = (inputA: string, inputB: string) => {
  const somecondition = inputA === inputB;
  return somecondition ? [false, 'error'] : [true, ''];
};

const payload: FormItemPayload = {
  value: 'bla bla',
  errorMsg: '',
  error: tuppleFunc('a', 'a').at(0),
};

the function will always return a type of array as below whose type is (boolean|string)[]

[false, 'error string dummy'] 
// or 
[false, 'error string dummy'] 

for reference attaching the typescript logs

Type 'string | boolean | undefined' is not assignable to type 'boolean'.
  Type 'undefined' is not assignable to type 'boolean'.ts(2322)
index.tsx(41, 5): The expected type comes from property 'error' which is declared here on type 'FormItemPayload'

My question

1. function clearly says that the first element will be on type boolean and second to be string.
2. at index 0 it will always be boolean so why is typescript complaining that it can be undefined?
3. my doubt is around the type (boolean|string)[] but i dont understand it very well.
4. i understand that it can be an array with boolean and string but unsure how to say at what index will it be
5. i also tried with giving the type [boolean,string] but no luck