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 fieldIsPtr {
-			field.Set(reflect.ValueOf(object))
-		} else {
-			field.Set(reflect.ValueOf(object).Elem())
+		if object != nil && field.CanSet() {
+			objectVal := reflect.ValueOf(object)
+
+			if fieldIsPtr {
+				field.Set(objectVal)
+			} else {
+				field.Set(objectVal.Elem())
+			}
 		}
 
 	}