The zip file was empty while Theme Installation

[ Moderator note: moved to Fixing WordPress. ]

I came across the following website to upload my own theme.

Submit Your Theme or Theme Update to the Directory

And I was ended up with the following error.

The zip file was empty.

But I my Zip File is not empty at all!

I Googled the above error but I could not find any helpful solution.

Does anyone know what the problem is?!