Fill: SelectCommand.Connection property has not been initialized

[dougancil]

I have the following code for a web page that I'm working on:

Imports System.Data.SqlClient
Imports System.Data
Partial Class _Default
    Inherits System.Web.UI.Page
    Protected Sub whosoncallButton_click(ByVal sender As Object, ByVal e As System.EventArgs) Handles whosoncallButton.Click
        Dim dt As New DataTable
        Dim da As New SqlDataAdapter
        Dim cmd As New SqlCommand
        Dim connectionString As String = "Initial Catalog=mdr;Data Source=xxxxx;uid=xxxxx;password=xxxxx"
        Dim con As New SqlConnection(connectionString)
        con.Open()
        cmd.CommandType = CommandType.StoredProcedure
        cmd.CommandText = "getoncall"
        cmd.Parameters.AddWithValue("@subschedule", TextBox1.Text)
        Try
            da.SelectCommand = cmd
            da.Fill(dt)
            GridView1.DataSource = dt
            GridView1.DataBind()
            con.Dispose()
        Catch ex As Exception
            Response.Write("Error:" & ex.ToString)
        End Try
    End Sub

    Protected Sub clearButton_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles clearButton.Click
        TextBox1.Text = ""
    End Sub
End Class

when the buttonclick event fires I receive the following error:

Error:System.InvalidOperationException: Fill: SelectCommand.Connection property has not been initialized. at System.Data.Common.DbDataAdapter.GetConnection3(DbDataAdapter adapter, IDbCommand command, String method) at System.Data.Common.DbDataAdapter.FillInternal(DataSet dataset, DataTable[] datatables, Int32 startRecord, Int32 maxRecords, String srcTable, IDbCommand command, CommandBehavior behavior) at System.Data.Common.DbDataAdapter.Fill(DataTable[] dataTables, Int32 startRecord, Int32 maxRecords, IDbCommand command, CommandBehavior behavior) at System.Data.Common.DbDataAdapter.Fill(DataTable dataTable) at _Default.whosoncallButton_click(Object sender, EventArgs e) in L:\wwwroot\WebSite1\Default.aspx.vb:line 17

this is line 17:

 da.Fill(dt)

and I can't figure out what this error message means. Can someone please assist me on what this error message is?

Thank you

Doug