diff --git a/retrieval/helpers_test.go b/retrieval/helpers_test.go
index 1b74b2ea4..d19f6b41f 100644
--- a/retrieval/helpers_test.go
+++ b/retrieval/helpers_test.go
@@ -21,7 +21,8 @@ import (
 
 type nopAppender struct{}
 
-func (a nopAppender) Append(*model.Sample) {
+func (a nopAppender) Append(*model.Sample) error {
+	return nil
 }
 
 func (a nopAppender) NeedsThrottling() bool {
@@ -33,13 +34,14 @@ type collectResultAppender struct {
 	throttled bool
 }
 
-func (a *collectResultAppender) Append(s *model.Sample) {
+func (a *collectResultAppender) Append(s *model.Sample) error {
 	for ln, lv := range s.Metric {
 		if len(lv) == 0 {
 			delete(s.Metric, ln)
 		}
 	}
 	a.result = append(a.result, s)
+	return nil
 }
 
 func (a *collectResultAppender) NeedsThrottling() bool {