Any case where `TError` can't be boxed in example?

public class ParseResult<TResult, TError>
{
    private readonly TResult? _result;
    private readonly TError? _error;

    private ParseResult(TResult? result, TError? error)
    {
        _result = result;
        _error = error;
    }

    public static ParseResult<TResult, TError> CreatedResult(TResult result)
        => new ParseResult<TResult, TError>(result, null);
}