listGrid.startEditingNew + cannot read property xx of null

[Years9000]

Respected everyone, I'm in desperate need of help.
In my darasource-countryDS, I used a field as primarykey like followed:
fields : [{name : "countryCode", title : "Code", primaryKey : true},...]
Then I defined a ListGrid-countryList using countryDS as the datasource.
When I call the function countryList.startEditingNew({countryCode:0}), I got a exception as followd:
Uncaught TypeError: Cannot read property 'countryCode' of null.
I know that is because I didn't initiate the value for primaryKey. But I don't know how to fix it.
Could anyone help me?! tks a lot.