I'm experiencing a problem with inkscape saves (v0.46) - maybe I'm doing something wrong?

Saving as svg is fine, but there is no option to save as png, it's not listed on the save as type list. There is an option to 'guess from file extension', and I have added .png to the filename, chosen 'guess from file extension and saved.

However, there is no thumbnail generated when I browse for the png in Explorer, and my image browser (Xnview) can't see it. When I try to open the png again in Inkscape I get a runtime error and the program crashes, but Gimp opens the png with no problems. If I then do a Save As from Gimp, the png saves normally, I can see it in my browser, and Inkscape opens it.

Any ideas what is happening? I don't want to have to use Gimp to open and resave every png I produce with Inkscape.