PrincipalContext throwing object reference not set to instance

Question

 var path = abcdev.xyz.com:636;
 PrincipalContext context = new PrincipalContext(ContextType.Domain, path)

 Error:Object reference not set to an instance of an object.

   at System.DirectoryServices.AccountManagement.PrincipalContext.ReadServerConfig(String serverName, ServerProperties& properties)
   at System.DirectoryServices.AccountManagement.PrincipalContext.DoServerVerifyAndPropRetrieval()
   at System.DirectoryServices.AccountManagement.PrincipalContext..ctor(ContextType contextType, String name, String container, ContextOptions options, String userName, String password)
   at System.DirectoryServices.AccountManagement.PrincipalContext..ctor(ContextType contextType, String name)
   at Authenticate(String userName, String password) in c:~\LdapRepository.cs:line 43
   at xyz.Web.LdapController.Login(LdapViewModel model) in C:~\Controllers\LdapController.vb:line 29

The PrincipalContext is working fine for all the clients except one ..I am able to connect to the path = abcdev.xyz.com:636 using Softerra LDAP but not sure why its throwing null object reference ...I have tried abcdev.xyz.com:636/dc=xyz,dc=com -- error server cannot be connected

The url given by client -- ldaps://abcdev.xyz.com:636 The client has verified tht our IP is white Listed So not sure what different should I try for this one client.