mismatch in UserServices::loginUser: Expected <type 'unicode'> recieved <type 'str'>

I get this error when i execute this method:

 @ladonize(int,PORTABLE_STRING,rtype=PORTABLE_STRING)
 def loginUser(self,userid,userpass):

mismatch in UserServices::loginUser: Expected <type 'unicode'> recieved <type 'str'>

I'm testing all the methods from validwsdl.com and i have some others accepting PORTABLE_STRING parameters and they just work fine, but this one gives me the unicode/str error. Even the methods where i accept one parameter that's a custom type formed by other types including integers and strings is working fine, so i cannot figure what's going wrong... The only difference is that i use the incomming string to do a comparation like this: if userpass == somepass: ...

but before comparing i use unicode function to turn them both to unicode so in the comparation they are both unicode.