How to make default value of datagridviewcell blank

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VB

validation

DataGridView

right now I have a vb.net program that reads in an excel file and displays the contents into a datagridview. My goal is to have code that tells the user if there is any value besides 1 or blank in the cell. Then I want to supply a default value of blank to the datagridviewcell. I was easily able to do all of this if the default value was 1 but when I change it to blank (String.empty) the program will keep the invalid input. Here is my code below. If anyone can figure out how to make the program supply a blank value as default value I would greatly appreciate it! :)

Sub validateDGV(rowindex, columnindex)

    Dim value As String = DataGridView1.Rows(rowindex).Cells(columnindex).Value.ToString
    If (columnindex = 1) Then
        Dim cellData = DataGridView1.Rows(rowindex).Cells(columnindex).Value
        If cellData Is Nothing OrElse IsDBNull(cellData) OrElse cellData.ToString = String.Empty Then
            'Do nothing because this is allowed
        ElseIf cellData <> 1 Then
            MessageBox.Show("Value must be 1 or Blank")
            DataGridView1.Rows(rowindex).Cells(columnindex).Value = String.Empty 'This should be supplying the default value of blank back to my datagridview but its not :(
            Exit Sub

        End If
    End If
End Sub