Confusing error message when argument title is missing in expect_doppelganger

[AlbanSagouis]

When expect_doppelganger is called with the plot object only, I find the error message confusing:

R> vdiffr::expect_doppelganger(p)
Error in `str_standardise()`:
! is_scalar_character(s) is not TRUE

Maybe

stopifnot("title argument is missing" = !is.null(title))

or

title <- match.arg(title)

would help users?