Whoever vs. Whomever

I came across this sentence in a text I was issued for a seminar: "Give the package to (whoever, whomever) comes to the door." The correct response was listed as "whomever."
The rationale for this was: substitute "he" or "him," as in "give the package to him." Therefore, in this sentence, "whomever" is correct.

I called a local community college that has a grammar hotline, and their response was that "whomever" was correct because it was the object of the proposition "to." I then asked the hotline person what function "comes to the door" served in this sentence, and she responded that it modified whomever!

Please, someone back me up here. "Whoever comes to the door" is a noun clause that is the object of the preposition "to," actually functioning as an indirect object in this sentence. Within the clause, the subject is "whoever." By virtue of the fact that it is the subject, it has to be in the nominative case - whoever, rather than whomever.