DataGridViewComboBoxColumn ignores DataSource string array

With VS2012, created DataGridView with six DataGridViewTextBoxColumn, all read only, all work fine.

Added seventh column in VS2012 designer as DataGridViewComboBoxColumn, not read only.

Set DataSource in C# form constructor as follows:

int row = dataGridView_ActiveJob.Rows.Add(view.viewName, view.cameraName, view.imageSize, view.frameRate.ToString(), mainForm.MakeExposureTime(view.exposureTime), "0%");
DataGridViewComboBoxCell cell = (DataGridViewComboBoxCell)(dataGridView_ActiveJob.Rows[row].Cells["Rotate"]);
cell.DataSource = new string[] { "0", "90", "180", "270" };

Form displays properly, showing Rotate column as combo box, but first click of combo box has no reaction, second click has drop down of one row blank, none of the four items in the above string array.

What is preventing combo box from dropping and showing the four string array options?