probably you'll need an update. Here's a quote from Melabs Website
When using WRITECODE to store data in the code space of these parts, you have to use a block-write method. This different than some of the other self-programming devices like the 16F87x, which will allow you to write a single location. Refer to the Microchip datasheet for more information.

The 18F parts have recently been introduced by Microchip. As with all new technology, they are not perfect. We have corrected for all the anomalies that we can in our development tools, but there is a chance that you may find a problem that affects your application. It's a good idea to check the Microchip website for current posting of errata concerning these parts
At this link