Conditionally Hide Columns From C# Code Behind

Question

I am attempting to conditionally hide columns in an asp:gridview based off selections of a checkboxlist. I have the below syntax, but this line

grdTest.Columns[othergrid5].Visible = item.Selected;

Is throwing an error of:

Index was out of range. Must be non-negative and less than the size of the collection. parameter name:index

What do I need to do to remedy?

protected void btnDoTheWork_Click(object sender, EventArgs e)
{
    var columnname = string.Empty;

    foreach (System.Web.UI.WebControls.ListItem item in cbxUno.Items)
    {
        columnname = item.Text;

        var othergrid5 = GetColumnIndex(grdTest, columnname);
        grdTest.Columns[othergrid5].Visible = item.Selected;
    }
}


private int GetColumnIndex(GridView grid, string ColName)
{
    foreach (DataControlField col in grid.Columns)
    {
        if (col.HeaderText.ToLower().Trim() == ColName.ToLower().Trim()) { return grid.Columns.IndexOf(col); }
    }
    return -1;
}