Back to when I had dialup, my browsers, if I didn't use a custom download manager like DownThemAll, tend to abruptly claim that the download's complete, but didn't download everything needed to actually make it complete.
Like, for example, a 60MB file would download at least 7MB before the browser declared the file downloaded completely and then stop downloading some more.
This would result in the file being "corrupt".
Try redownloading the zip file.
This happens to me too, usually when I download large files. I just had to download it over and over until it worked.