Setting a value to null in grid won't insert column's default value on save

When I Insert a new row in the grid, all the values are null. I have autoincrementing primary key. If I copy all values from a different row into this new row, except leaving the primary key null, when I save, I get a row with a new key.

However, when I duplicate an entire row in the grid and then use the "Set to NULL" command to set the key to "(null)", I get this error: 

java.sql.SQLException: Cannot insert explicit value for identity column in table 'vtorderitem' when IDENTITY_INSERT is set to OFF.

It seems like the column is being set to a string instead of actually null.

Update: I looked at the SQL in the error message and it showed a normal null. I don't know what is wrong. 

INSERT INTO "dbo"."vtorderitem" ("id", "orderid", "productkey", "amount", "quantity") VALUES (null, 20436, '0000KG0000ES4', 66610000.00, 1.00)

 This is MSSQL database.

I don't see an "edit" button on my posts, but wanted to clarify that the error in the original post is thrown when I hit Save, not when I issue the Set to NULL command.