Blender crashes after undo (ctrl + z) a soft cut or hard cut on a strip using a custom proxy file.

AND

If you don't undo and instead delete a part of the strip, blender disables the proxy and you need to save and restart blender to have the proxy working again.

CRASH #1:
* Load movie strip
* Enable proxy and load a custom proxy strip.
* Set proxy render to "Proxy size 50%"
* Hard cut or soft cut a part of the strip
* press ctr + z to undo
* CRASH!

CRASH #2:
* Same steps as crash #1 but don't press ctr + z (undo)
* After the hard cut or soft cut delete a part of the strip
* click in some part of the remaining strip (the proxy is gone in the preview)
* Save and close blender (or only close blender)
* CRASH!

Video files used:

Original videofile:
* Avi file
* size: 1280 x 720
* codec: h.264

Proxy file:
* Avi file
* size: 640 x 360
* codec: MJPG (JPG)

---
SYSTEM:
Windows 7 ultimate 64 bits.
Blender 2.56a, 32 bits (2.56.0 r34076, official release)
Nvidia 7800GT (256 MB)
2GB RAM
3 GHz Intel Dual core (E8400) processor.

Sorry for my english (I speak spanish)