React hook form useFormContext generating "argument of type 'string | undefined'" error

I am new to react and I am using react-hook-form useFormContextand it is generating the following error, Argument of type 'string | undefined' is not assignable to parameter of type 'string'. Type 'undefined' is not assignable to type 'string'. and I am not sure why. My code is the following,

const mapFields = new Map([['d1', '1'], ['d1', '2'], ['d3', '3']]); register(mapFields.get('d1'), { required: true })

I tried passing a function into register instead of a map and it worked fine so not sure why this is occurring.

>Solution :

You can use the non-null assertion operator (!) to tell TypeScript that the value cannot be undefined.

register(mapFields.get('d1')!, { required: true })