MongoDB C# string comparison

Question

I tried to do string comparison using MongoDB C# driver, However, I always got error message as following:

Unable to determine the serialization information for the expression:

y.strVal.CompareTo("15.5")

var q2 = from objb in q1.AsQueryable<M>()
         where objb.lista.Any(y=> y.strVal.CompareTo(tempC.ToString())>=0)
         select objb;