callproc type casts

I've got a plpython function, with a character varying param.
I can call it from sql.

But when i try to call it with psycopg2.callproc('testfunc', ['test']),
i've got the error:
function testfunc(unknown) does not exist
HINT:  No function matches the given name and argument types. You might
need to add explicit type casts.

I'm using postgres 8.4.4 and psycopg2 2.0.14

Thanks in advance:
Imre Horvath