Linq ToDictionary Not Defined?

I have this code

var contacts = dr.mktDoctorContacts
    .GroupBy(x => x.ContactType)
    .Select(zb => new 
     { 
         Key = zb.Key,
         GroupWiseContacts = zb.Select(x => x.Contact).ToList()
     })
    .ToDictionary<string,List<string>>(y => y.Key, y => y.GroupWiseContacts)

I don't know what is wrong with this code.

Compile time error msg says:System.Generic.IEnumerable does not contain definition of and best extension method overloads has some invalid arguments. i can see only two overloads of ToDictionary Method in my visual studio tooltip sort of documentation whereas i have come across more than two overloads of ToDictionary on the web
Edit Here is exact Error message at compile time

Error 13 'System.Collections.Generic.IEnumerable' does not contain a definition for 'ToDictionary' and the best extension method overload 'System.Linq.Enumerable.ToDictionary(System.Collections.Generic.IEnumerable, System.Func, System.Collections.Generic.IEqualityComparer)' has some invalid arguments