P: Merging all layers to background makes layer unreceptive for changes if Generator is enabled

Photoshop CC 14.2: To replicate this issue:
- Create a new image with white background.
- Paint something
- Add a new layer
- Paint something more on this layer
- Merge down
- Try to paint something on the background layer and the program spits out a generic software error (In swedish "Det gick inte att använda penseln därför att ett programfel uppstod").

This happens sometimes even when I have tree layers where i merge to bottom two, but then the top layer becomes unreceptive.

Workarounds:
Add a new layer and remove the same.
If the problem occurs on the background layer: make it a regular layer.

Setup:
Mac OS X 10.9.1
Photoshop CC