When I try to add a user in security rolesI get this error:

Error: is currently unavailable.
DotNetNuke.Services.Exceptions.ModuleLoadException: String reference not set to an instance of a String. Parameter name: name ---> System.ArgumentNullException: String reference not set to an instance of a String. Parameter name: name at System.Globalization.CultureInfo..ctor(String name, Boolean useUserOverride) at DotNetNuke.Security.Roles.RoleController.SendNotification(UserInfo objUser, RoleInfo objRole, PortalSettings PortalSettings, UserRoleActions Action) at DotNetNuke.Security.Roles.RoleController.AddUserRole(UserInfo objUser, RoleInfo objRole, PortalSettings PortalSettings, DateTime effDate, DateTime expDate, Int32 userId, Boolean notifyUser) at DotNetNuke.Modules.Admin.Security.SecurityRoles.cmdAdd_Click(Object sender, EventArgs e) --- End of inner exception stack trace ---

When I try again it add the user to the role but getting this error when I click first time. I am using DNN 5.1 .