Unity: Reference to type 'SecureString' claims it is defined in 'mscorlib'

[uelipeter]

I built the WindwosDevicePortalWrapper and added it to my unity project (Scripting Runtime .Net 4x, Api Compatibility Level .Net 4.x). When I build my unity project I get the following error:
Reference to type 'SecureString' claims it is defined in 'mscorlib'
for the following line of code:
portal = new DevicePortal( new DefaultDevicePortalConnection( "https://127.0.0.1", "username", "password"));

Can anyone help me with this?