If anyone is still looking (like I did)...
In my case, it was data type mismatch (even though I had int's, it was still treating them as strings) since DGV wasn't bound. My solution was to assing .ToString() on the objects in the combobox and during the assignment for default values.
if (dgv.Columns["Number"].GetType() == typeof(DataGridViewComboBoxColumn))
DataGridViewComboBoxColumn col = (DataGridViewComboBoxColumn)dgv.Columns["Number"];
for (int i = 0; i < 100; i++)
row.Cells["Number"].Value = "1";