[plt-scheme] Are booleans s-expressions?

I'm using PLT scheme 4.2.3 on windows xp. I'm using the intermediate
language with lambda and the 2htdp/universe teachpack.

The documentation says that booleans are s-expressions but the sexp?
function doesnt seem to recognize that. Am I using it wrong?

> (sexp? true)
false
> (sexp? false)
false

http://download.plt-scheme.org/doc/4.2.3/html/teachpack/2htdpuniverse.html#%28tech._s._expression%29

S-expression An S-expression is roughly a nested list of basic data;
to be precise, an S-expression is one of:

    *     a string,
    *     a symbol,
    *     a number,
    *     a boolean,
    *     a char, or
    *     a list of S-expressions.