Funny Sign

I was walking past a classroom where a Canadian teacher was giving her students a midterm and almost passed out when I saw this instruction written on the black board in big clear letters:

Put nothing on desk

(penis okay)

Um, did she mean, "A pen is okay"?

I asked in passing if there was a bias against female genitalia in general or if ANY penis would do and she glanced at me, at the sign, and then said, "Huh?" After a moment or two she laughed, and added a bit more space between the "pen" and "is." 

I probably wouldn't have corrected it, but she's a far nicer women than I am.