Wrong type argument: stringp, nil error

I need to use update-directory-autoloads function in a little el-script. When I'm trying to call this function with an argument that is a name of a directory I receive this error: Wrong type argument: stringp, nil.
Call looks like this:
(update-directory-autoloads "~/test")

The problem was solved. If somebody else will need it solution is here:
http://stackoverflow.com/questions/1476 ... -nil-error