Set a field to blank datetime in screen customization

I’m trying to set a custom date on partplant to null from a screen customization.
I’m getting an error:
String was not recognized as a valid DateTime.

I’ve tried:
edvPartPlant.CurrentDataRow["QASIssueDate_c"] = string.Empty; edvPartPlant.CurrentDataRow["QASIssueDate_c"] = null; edvPartPlant.CurrentDataRow["QASIssueDate_c"] = "";

I think it is

edvPartPlant.CurrentDataRow["QASIssueDate_c"] = DBNull.Value;