nil lookup fix

lookup.go

@@ -156,10 +156,14 @@ func Lookup(db *mgo.Database, structField string, results interface{}, selector
 		object := objectIDs[objectID]
 
 		field := elV.FieldByName(structField)
+		if object != nil && field.CanSet() {
+			objectVal := reflect.ValueOf(object)
+
 			if fieldIsPtr {
-			field.Set(reflect.ValueOf(object))
+				field.Set(objectVal)
 			} else {
-			field.Set(reflect.ValueOf(object).Elem())
+				field.Set(objectVal.Elem())
+			}
 		}
 
 	}