DataSource Controls :: Default Value Or Binding With Newid?

I've created a table with a userId column of type uniqueidentifier, and have set that column as the primary key. In the column properties, under the Default Value or Binding, I've put (newid()). I thought that this would create a new uniqueidentifier if no value is specified in the insert statement.

However, when I try to add a new row in the table data view and leave the userId blank, it does not let me add the row. It gives me an error saying that the value cannot be NULL (since it is a primary key), however I had deliberately left it NULL so that it would add a new value by calling newid(). Have I made a false assumption?