TexSubImage to bound pbuffers?

This is what I’m doing:

*Creating a pbuffer and filling it with data
*Binding it to a texture using wglBindTexImageARB()
*Updating a subregion of the texture with glTexSubImage2D()
*Retrieving the results from the card with glGetTexImage()

The subregion gets updated correctly, but anything down or to the left of the updated region becomes black. ie. is corrupted.

Has anyone else experienced problems like this?

The pbuffers are all GL_NV_TEXTURE_RECTANGLE 8bpc images. I’m working with a FX5600 with latest drivers.