Could not load assembly 'System.Web' error

Building Xamarin.Android project ends with:

/Users/yoorek/Dropbox/Code/VisualStudio/Synchronizer/Android/Android.csproj (Build) ->
/Library/Frameworks/Mono.framework/External/xbuild/Xamarin/Android/Xamarin.Android.Common.targets
(_ResolveAssemblies target) ->
/Library/Frameworks/Mono.framework/External/xbuild/Xamarin/Android/Xamarin.Android.Common.targets: error :
Exception while loading assemblies: System.IO.FileNotFoundException:
Could not load assembly 'System.Web, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'.
Perhaps it doesn't exist in the Mono for Android profile?

I have no clue...