"Specified cast is not valid" error

I am getting the following error: "Specified cast is not valid"

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Private Sub cmdCalcTotal_Click(ByVal sender As System.Object, ByVal e As
System.EventArgs) Handles cmdCalcTotal.Click


Dim ds As DataSet

Try

ds = CType(dgrOrderDetails.DataSource, DataSet)

'Other code here to calculate the order total

Catch ex As Exception

MessageBox.Show(ex.Message, "Cannot set the dataset to the datagrid
datasource")

End Try

End Sub

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Suggestions are greatly appreciated.

Have you assigned a DataSet object to the DataSource property of the grid in the first place? If you've assigned a DataTable or a BindingSource or something else then you cannot cast it as type DataSet.