DataColumn Add

I have this statement where custDs is the dataset I populate and then am trying to add a column of string type to the

custds.tables(0).Columns.Add("Total",system.type.gettype("system.string"))

but im getting this error on runtime,any ideas since my syntax or order is

System.ArgumentNullException: 'dataType' argument cannot be null. Parameter name: dataType at System.Data.DataColumn..ctor(String columnName, Type dataType, String expr, MappingType type) at System.Data.DataColumnCollection.Add(String columnName, Type type) at ASP.moiz_aspx.Button1_Click(Object sender, EventArgs e)

cn.Open()

				Dim sqlstring as String ="select * from persons"

	Dim sql1command as sqlcommand 
				sql1command = new sqlcommand(sqlstring,cn)
				Dim custDA As SqlDataAdapter = New SqlDataAdapter("select * from persons", cn)
				Dim custDS As DataSet = New DataSet()

				custDA.Fill(custDS, "persons")

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