A function to return a dataset only working in first loop

I use a function to return dataset in which passing a store procedure and MereberID. 
It works fine for calling one time but once in for loop, it only works for first loop. 
When i=0, it works but when i=1 (second loop), an error occurred: "Object reference not set to an instance of an object"
How to fix it?
dim myds as new dataset
 For i As Integer = 0 To listboxmember.Items.Count - 1
          myds = GetSPDataSet("mystoreprocedure", listboxmember.items(i))
          'some codes here'
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Public Function GetSPDataSet(ByVal SPName As String, ByVal _membid As String) As DataSet Dim MyConnection As SqlConnection Dim MyCommand As SqlCommand Dim MyDataSet As New DataSet Dim MySQLDataAdapter As SqlDataAdapter
MyConnection = MYSQLCONNECTION MyCommand = New SqlCommand(SPName, MyConnection) MyCom ...