NetworkCredential error in ASP.NET

Question

I am trying to access a webpage through ASP.NET using the NetworkCredential class. However I keep getting an exception with the following message System.Security.Cryptography.CryptographicException: The handle is invalid

Below is my code on how I am trying to call the function. Any help is greatly appreciated.

C#:

System.Net.WebClient client = new System.Net.WebClient();
client.Credentials = new System.Net.NetworkCredential("Admin", "Nimda");

Stack Trace

[CryptographicException: The handle is invalid.
]

System.Security.SecureString.ProtectMemory() +154
   System.Security.SecureString.InitializeSecureString(Char* value, Int32 length) +170
   System.Security.SecureString..ctor(Char* value, Int32 length) +65
   System.Net.SecureStringHelper.CreateSecureString(String plainString) +6181188
   System.Net.NetworkCredential..ctor(String userName, String password) +64