I updated the PR it is a bit more complex but the message should now be even clearer.

If the error comes from a setter method you will get something like this

Fatal error: Uncaught League\Csv\Serializer\TypeCastingFailed: Casting the method setReview() first argument foobar using the record field review failed; The null value can not be cast to a string; the property type is not nullable.

if an offset is used instead you will get the following:

Fatal error: Uncaught League\Csv\Serializer\TypeCastingFailed: Casting the property tags using the record field offset 5 failed; The null value can not be cast to a string; the property type is not nullable.

The current message is no longer changed but it is prefixed with a more detail information about the context around the type casting failure.