Woops, It was late when I wrote that.. The actual situation is that in the Packages file installed-size is spelt 'installed-size' and in the status file it is 'Installed-Size'.
It seems dpkg-scanpackages is what causes this, Ben: can you find out what does this, I would like to submit a patch with the bug report so there is a chance of getting it fixed, I would also like to fix the version on master so that it IS fixed :P I'll just add an extra check for the other spelling and try to track down what does this. Jason

