Visual Studio - Building release iOS project with web reference

I'm using Visual Studio 2017. I have a Xamarin forms project and a iOS project. I'm adding a "Web reference" to the iOS project and using the dependency service to work with web services. However, I'm getting the following error when I try to build my iOS release project:

1>SGEN : error : Não foi possível carregar arquivo ou assembly 
'mscorlib, Version=2.0.5.0, Culture=neutral, 
PublicKeyToken=7cec85d7bea7798e' ou uma de suas dependências. Falha 
na validação de nome forte. (Exceção de HRESULT: 0x8013141A)

Steps to reproduce the error: - Create new VS Xamarin forms project. - Fill all provisioning information. - Build iOS project as Release Iphone -> no error, it builds! - Simply add a "Web reference" to the iOS project. It returns that it's not possible to load the assembly mscorlib, Version=2.0.5.0.

Any ideas on that? Thank you!