UserPrincipal.FindByIdentity Permissions

Question

I'm attempting to use the .NET System.DirectoryServices.AccountManagement library to obtain the UserPrincipal for a particular Active Directory user.

I've got the following code:

PrincipalContext context = new PrincipalContext(ContextType.Domain, "DomainName");
userPrincipal = UserPrincipal.FindByIdentity(context, IdentityType.SamAccountName, username);

This code is running as a valid domain user, but when I execute it I get the following exception:

System.DirectoryServices.DirectoryServicesCOMException (0x8007052E): Logon failure: unknown user name or bad password.

What's interesting is that I can make the following call, using the same context, without a problem:

context.ValidateCredentials(username, password, ContextOptions.Negotiate)

Ideas?