Accessing the LDAP with PrincipalContext to a public server

Question

I am having issues connecting to a public LDAP using PrincipalContext. I can get directory entry working fine:

string sDomain = "LDAP://ldap.itd.umich.edu:389";
string sDefaultOU = "ou=System Groups,ou=Groups,dc=umich,dc=edu";
string sServiceUser = "cn=Directory Manager,o=University of Michigan,c=us";
string sServicePassword = "";

DirectoryEntry de = new DirectoryEntry(sDomain + "/" + sDefaultOU, sServiceUser, sServicePassword, AuthenticationTypes.ServerBind);
DirectorySearcher ds = new DirectorySearcher(de);
SearchResult sr = ds.FindOne();

But when I try to do the same thing with PrincipalContext I get a null reference error:

sDomain = "ldap.itd.umich.edu";
PrincipalContext oPrincipalContext = new PrincipalContext(ContextType.Domain, sDomain, sDefaultOU, sServiceUser, "");

Any ideas what I am doing wrong?