diff --git a/server/rest.go b/server/rest.go
index 83a631ea5..f682c0011 100644
--- a/server/rest.go
+++ b/server/rest.go
@@ -1872,7 +1872,7 @@ func (s *RestServer) getTypedUserItemFeedback(request *restful.Request, response
 	itemId := request.PathParameter("item-id")
 	if feedback, err := s.DataClient.GetUserItemFeedback(ctx, userId, itemId, feedbackType); err != nil {
 		InternalServerError(response, err)
-	} else if feedbackType == "" {
+	} else if feedbackType == "" || len(feedback) == 0 {
 		Text(response, "{}")
 	} else {
 		Ok(response, feedback[0])
diff --git a/storage/cache/redis.go b/storage/cache/redis.go
index 68265b7e2..888baf3bb 100644
--- a/storage/cache/redis.go
+++ b/storage/cache/redis.go
@@ -475,6 +475,9 @@ func decodeCategory(s string) (string, error) {
 	if err != nil {
 		return "", errors.Trace(err)
 	}
+	if len(b) == 0 {
+		return "", nil
+	}
 	return string(b[1:]), nil
 }
 
@@ -496,6 +499,7 @@ func decodeCategories(s string) ([]string, error) {
 		if err != nil {
 			return nil, errors.Trace(err)
 		}
+		// category = "" is also a valid category
 		categories = append(categories, category)
 	}
 	return categories, nil