Error MT2101: Can't resolve the reference 'System.String System.String

I Used FF Image loading in Xamarin Forms.App is working in Android Properly but giving error in iOS.Please suggest me for this.
Error MT2101: Can't resolve the reference 'System.String System.String::Trim(System.Char)', referenced from the method 'System.Threading.Tasks.Task`1<FFImageLoading.Work.DataResolverResult> FFImageLoading.DataResolvers.BundleDataResolver::ResolveFromBundlesAsync(System.String,System.String,FFImageLoading.Work.TaskParameter,System.Threading.CancellationToken)' in 'mscorlib, Version=2.0.5.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e'. (MT2101) (SwimmingSpot.iOS)