Unzip file with Xamarin

Hi! I'm trying to unzip a file with this methods:

ZipFile.ExtractToDirectory (localPath, outputDir); //found on this site: http://www.codeproject.com/Articles/381661/Creating-Zip-Files-Easily-in-NET

I'm using api System.IO.Compression;
If I use this api for zip and unzip a file all is ok but when I want to unzip a file download from a website I found this Exception both for iOS and Android:

System.IO.EndOfStreamException: Failed to read past end of stream.

Someone could help me to avoid this problem?

Thanks