I want to add a new user into a role, but got the following error: 

Error: User Roles are not currently available.
DotNetNuke.Services.Exceptions.ModuleLoadException: The specified string is not in the form required for a subject. ---> System.ArgumentException: The specified string is not in the form required for a subject. at System.Net.Mail.Message.set_Subject(String value) at System.Net.Mail.MailMessage.set_Subject(String value) at DotNetNuke.Services.Mail.Mail.SendMail(String MailFrom, String MailTo, String Cc, String Bcc, MailPriority Priority, String Subject, Mailformat Bodyformat, Encoding BodyEncoding, String Body, String Attachment, String SMTPServer, String SMTPAuthentication, String SMTPUsername, String SMTPPassword) at DotNetNuke.Services.Mail.Mail.SendMail(String MailFrom, String MailTo, String Bcc, String Subject, String Body, String Attachment, String BodyType, String SMTPServer, String SMTPAuthentication, String SMTPUsername, String SMTPPassword) at DotNetNuke.Modules.Admin.Security.SecurityRoles.SendNotification(Int32 UserId, Int32 RoleId, String Action) at DotNetNuke.Modules.Admin.Security.SecurityRoles.cmdAdd_Click(Object sender, EventArgs e) --- End of inner exception stack trace ---

the steps:

1.        Admin -> Roles

2.       Click ‘manager users’

3.      click 'Add new user', then page redirect to the above error,

Is there any fix ?  thanks