Error on Thror's Map in The Hobbit

I was just wondering if anyone else has noticed the error on Thror's Map on the second line of Runes in the centre of the map. It's under the writing "The Desolation of Smaug" and the letter W (which resembles a pointy letter P) and the letter H (which looks like an H with two lines) are swapped by accident. The word there should be "when" but the word that's written is actually "hwen." So the entire thing reads "Stand by the gray stone hwen the thrush knocks and the setting sun with the last light of Durins (without an ' because there's no rune for that) Day will shine upon the keyhole."

Wonder if it's there on purpose or if it's just a typo of some sort? The letter W and letter H are correct through the rest of the writing there. Check it out in your copy of The Hobbit when you get a chance.