Forum .LRN Q&A: Error in creation User

I have a probelm, when I try to create a new user, show this error:

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Here is a detailed dump of what took place at the time of the error, which may assist a programmer in tracking down the problem:

can't use empty string as operand of "!"
while executing
"if { ![parameter::get_from_package_key -package_key dotlrn-portlet -parameter AllowCreateGuestUsersInCommunity] } {
if { ![dotlrn::a..."
("guestuser" arm line 2)
invoked from within
"switch $parameter {
"cenrollment" {
if { ![parameter::get_from_package_key -package_key dotlrn-portlet -parameter AllowChangeEnrollmentP..."
(procedure "dotlrn_portlet::is_allowed" line 4)
invoked from within
"dotlrn_portlet::is_allowed -parameter guestuser"
("uplevel" body line 39)
invoked from within
"uplevel {
#
# Copyright (C) 2001, 2002 MIT
#
# This file is part of dotLRN.
#
# dotLRN is free software; you can redistribute it and/or modi..."
(procedure "code::tcl::/var/lib/aolserver/dotlrn/packages/dotlrn/www/use..." line 2)
invoked from within
"code::tcl::$__adp_stub"
invoked from within
"if { [file exists $__adp_stub.tcl] } {

# ensure that data source preparation procedure exists and is up-to-date
adp_init tcl $__adp_stub
..."
("uplevel" body line 3)
invoked from within
"uplevel {

if { [file exists $__adp_stub.tcl] } {

# ensure that data source preparation procedure exists and is up-to-date
adp_init t..."
(procedure "adp_prepare" line 2)
invoked from within
"adp_prepare "
(procedure "template::adp_parse" line 30)
invoked from within
"template::adp_parse [file root [ad_conn file]] {}"
(procedure "adp_parse_ad_conn_file" line 5)
invoked from within
"$handler"
("uplevel" body line 2)
invoked from within
"uplevel $code"
invoked from within
"ad_try {
$handler
} ad_script_abort val {
# do nothing
}"
invoked from within
"rp_serve_concrete_file [ad_conn file]"
(procedure "rp_serve_abstract_file" line 60)
invoked from within
"rp_serve_abstract_file "$root/$path""
("uplevel" body line 2)
invoked from within
"uplevel $code"
invoked from within
"ad_try {
rp_serve_abstract_file "$root/$path"
set tcl_url2file([ad_conn url]) [ad_conn file]
set tcl_url2path_info..."

Help me plese.