Unable to change password in AD

Hello,

I am trying to change the password for an existing user in AD through the script below. However, I am getting an error "Server Unwilling to process request".

The script is running on a Windows 2003 domain controller. What am I doing wrong?

$userAD = [ADSI]"LDAP://localhost:389/CN=Test User,cn=Users,DC=iamdev,dc=local"
$userAd.psbase.invoke("SetPassword","passwd123")

thanks