New EXIF tags are always "Unknown tag"

This is how I add some custom EXIF tags to a image:

PropertyItem propItem = file.PropertyItems[0];
propItem.Id = //ID
propItem.Type = 2;
propItem.Value = //Value
propItem.Len = //Length
file.SetPropertyItem(propItem);

Example can be found here

But the tag does not have a name:

How can I add a name to a PropertyItem?