nested generic HashMap problem

I've looked at the Java generics tutorial, Langer's FAQ, and similar
online pages, but I'm still don't see what, if anything, I can do to
make the last line (marked with the comment) compile, other than do a
typecast and suppress the unchecked warning. Am I asking the impossible
or missing the obvious?

import java.util.HashMap;

public class Demo<T> {
   private Class<T> base;
   private Cache cache;

   Demo(Class<T> b, Cache c) { base = b; cache = c; }

   Class<T> getBaseClass() { return base; }
   T get(long id) { return cache.get(this, id); }
}

class Cache {
   private HashMap<Class<?>, HashMap<Long, ?>> maps
     = new HashMap<Class<?>, HashMap<Long, ?>>();

   public <T> T get(Demo<T> demo, long id) {
     return getMap(demo).get(id);
   }

   public <T> void add(Demo<T> demo) {
     maps.put(demo.getBaseClass(), new HashMap<Long, T>());
   }

   private <T> HashMap<Long, T> getMap(Demo<T> demo) {
     return maps.get(demo.getBaseClass()); // incompatible types
   }
}