C# – Setting DataGridView.DefaultCellStyle.NullValue to null at designtime raises error at adding rows runtime

In Visual Studio 2008

• add a new DataGridView to a form
• Edit Columns
• Add a a new DataGridViewImageColumn
• Open the CellStyle Builder of this column (DefaultCellStyle property)
• Change the NullValue from System.Drawing.Bitmap to null
• Try to add a new Row to the DataGridView at runtime
  (dataGridView1.Rows.Add();)
• You get this error: System.FormatException: Formatted
  value of the cell has a wrong type.

If you change back the NullValue to System.Drawing.Bitmap (as it was) you still get the same error at adding a row.

If you set the NullValue at runtime instead of designtime you don't get anny error.
(dataGridView1.Columns[0].DefaultCellStyle.NullValue = null;)

Could you tell me why is that?