Please fix part of the kata's description:

Each letter of a word scores points according to it's position in the alphabet: a = 1, b = 2, c = 3 etc.

"it's" -> "its"

As a number of comments below, I have experienced a problem with a random test, although basic tests were successfully passed. I used R to solve the program and received:
Test Failed
invalid 'type' (list) of argument
1: replicate(95, {
s <- character()
for (i in seq_len(sample(5:24, 1))) s <- paste(s, paste(sample(letters, sample(10,
1)), collapse = ""), collapse = " ")
expect_equal(high(s), pineapple_express(s))
}) at tests.R:20
2: sapply(integer(n), eval.parent(substitute(function(...) expr)), simplify = simplify)
3: lapply(X = X, FUN = FUN, ...)
4: FUN(X[[i]], ...)
5: expect_equal(high(s), pineapple_express(s)) at tests.R:29
6: quasi_label(enquo(object), label)
7: eval_bare(get_expr(quo), get_env(quo))
8: high(s)

Any ideas why that happen? I also intentionally converted my output field into a single string.

Same issue here:
expect_equal( most_frequent_digit_sum(17),9), while it should be 8.