I can not send message to mobile via amazon SNS in VB.NET

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VB

Amazon

i want to send the sms to the user via Amazon SNS .i wrote the following code in vb.net but error occured "The message attribute 'AWS.SNS.SMS.SMSType' has an invalid message attribute type, the set of supported type prefixes is Binary, Number, and String."....... i Don't know where the problem in the code.my code on button click event is:

What I have tried:

 Imports Amazon.SimpleNotificationService
 Imports Amazon.SimpleNotificationService.Model
 Public Class Form1
     Dim myclient As New AmazonSimpleNotificationServiceClient
     Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
        Dim snsreq As New PublishRequest
        Dim snsres As New PublishResponse
        Dim snsmsgstng As New Dictionary(Of String, MessageAttributeValue)()
        snsmsgstng.Add("AWS.SNS.SMS.SenderID", New MessageAttributeValue() With {.StringValue = "testing", .DataType = "string"})
        snsmsgstng.Add("AWS.SNS.SMS.SMSType", New MessageAttributeValue() With {.StringValue = "Promotional", .DataType = "string"})
        snsreq.MessageAttributes = snsmsgstng
        snsreq.PhoneNumber = "+972543784512"     'phone No
        snsreq.Message = "you get message ,if yes then tells me???.."
        snsres = myclient.Publish(snsreq)
     End Sub
End Class

i also add following in app.config

<appSettings  >
 <add  key="AWSAccessKey" value="MYAWSAccessKey" />
 <add key="AWSSecretKey" value="MYAWSSecretKey"/>
 <add key="AWSRegion" value="ap-southeast-1"/>
</appSettings>

please tell me where i should change the code...!!!