possible change/bug in NOT NULL?

In my previous version of clsql 5.1.3 the following:

   (clsql-sys::sql-output (clsql:sql-operation 'is (clsql:sql-expression :table "xtable" :attribute "yval") (clsql:sql-operation 'not-null)))

evaluates to 

=>      "(\"xtable\".\"yval\" IS NOT NULL)"

But in the latest one I downloaded (clsql 6.0.1), it gives


=>    "(\"xtable\".\"yval\" IS \"NOT NULL\")"   ;; note quotes around "NOT NULL"

This seems to be invalid sql syntax, judging from a failed query.

Apologies if I'm missing something.

J.Klein