[plt-scheme] if without third argument

AFAIK it is allowed (both in R6RS and R5RS), but mzscheme doesn't like it:

Welcome to MzScheme v4.0.1 [cgc], Copyright (c) 2004-2008 PLT Scheme Inc.
> (if 1 2 3)
2
> (if 1 2)
stdin::11: if: bad syntax (must have an "else" expression) in: (if 1 2)

 === context ===
/usr/lib/plt/collects/scheme/private/misc.ss:68:7

Why is this?

Sorry for my ignorance.

Tamas