Just came across this after trying the speckcart module out for the first time. I think your problem here is that you are trying to set the CartItemId. The database code (provided within the module) makes the cart item Id an auto-incremented primary key field, which therefore should not be set as the database should create it automatically.
Also, you must supply a quantity as the quantity's database field is set to not accept null values.
Other than that, everything should work.
Hope this helps.