There's no 'r' in Yom Kippur?

From The Encyclopedia of Jewish Humor by Henry D. Spalding.

It was Yom Kippur, holiest of fast days, and Simon Dorman, of Dorman and Litvak, Furriers, was on his way to shul. He passed by a sea-food restaurant and, to his chagrin, spied his partner inside.

Bursting though the entrance, he ran to the man's table and bellowed, "Litvak, how can you sit here and eat oysters on this day of all days? It's a shame and a disgrace!"

"What's a shame and a disgrace?" demanded Litvak. "There's no 'r' in Yom Kippur?"