It's not every day that Roman numerals are funny, is it? Actually, this reminds me of that classic joke by Wayne & Shuster which depends on the Latin construction of singular and plural nouns --
Roman: I'd like a martinus please.
Bartender: You mean a martini.
Roman: No, if I want two, I'll order two!
(All Canadians "of a certain age" remember Wayne & Shuster . . . .)