What steps will reproduce the problem?

1. make/load a non-quadratic dds image with mipmaps (for example aspect ratio 1:2)
2. try to save it
3. try to select "use existing mipmaps": its grayed out

What is the expected output? What do you see instead?

It should be possible to use existing mipmaps - to be able to correct distortions of mipmaps produced after a previous step of creating them automatically.

What version of the product are you using? On what operating system?

GIMP 2.8.10; GIMP-DDS 3.0.1

Please provide any additional information below.

This has been reported as debian bug #731486 https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=731486

The author of the bug report said it is due to "when exporting existing layers as mipmaps it validates the layers by looking for a chain with each successive layer being half the size of the previous, using a bitshift for a fast divide by 2, so when looking for the next layer after 2x1 it would try to find 1x0 and fail."