How can I check service bus relay exist or not

I try using "RelayExistsAsync()" of NamespaceManager. But , I received the following error message.

"System.ArgumentException(400)
Cannot send message because the service endpoint hosted at the specified address uses a netTcpRelayBinding that cannot be accessed over HTTP"

My code is as

string connectionString = CloudConfigurationManager.GetSetting("Microsoft.ServiceBus.ConnectionString");
var namespaceManager = NamespaceManager.CreateFromConnectionString(connectionString);

// Check relay already exist or not
bool res = await namespaceManager.RelayExistsAsync(relayName);

my connection string is

"Endpoint=sb://ServiceNameSpace.servicebus.windows.net/;SharedAccessKeyName=RootManageSharedAccessKey;SharedAccessKey=SecretKey"

Need help
Thanks in advance.. :)