[Solved] Alpha channel export broken?

I baked specular reflection to a texture with alpha channel. The result looks fine in the UV image editor (see here) but when I export it to PNG or TGA (with “RGBA” selected in the format panel), the result is completely transparent, i.e. all of the alpha channel is set to 0. Am I making a mistake? How can I export the texture properly to either PNG or TGA or some other format supported by the GIMP?

I’m using the official 2.46 build on Ubuntu Linux 32bit.