EdmFunctionAttribute not working in WPF

        [EdmFunction("VIS6Model.Store", "CacheField")]
        public static string CacheField(string s1, int i1)
        {
            throw new NotSupportedException("This function can only be used in a LINQ to Entities query");
        }

the usage looks like this:

      IEntityQuery<MasterEntityDb> enq = VisContext.VisRepository.Manager.GetQuery<MasterEntityDb>();
      enq = enq.OrderByDescending(x => MySqlLinqFunctions.CacheField(x.Cache, SortFieldIdx));

      var searchQuery = enq.Where(SearchPredicate).Take(PageSize);

      var results = (await VisContext.VisRepository.Manager.ExecuteQueryAsync(searchQuery)).ToList();

(VisContext.VisRepository.Manager is the DevForce Entity Manager)

It works fine in Silverlight but when we run in on the WPF Client we get the NotSupportedException from above.

Is this an known issue?