DDS converter: problem

I convert my bmps with the following settings:

save alpha (if any)
dxt1c
build mipmaps

Then I want to compress them with IFFSET but get the following error:

invalid image format (need 512x512, size=... fmt=DXT1]
What did I do wrong?

are there different dxt1 options besides your "c"?

i use the GIMP DDS plugin so my choices are just raw DXT1, DXT5, and if I want mipmaps or not. None of that "c" stuff