diff --git a/storage/metric/memory.go b/storage/metric/memory.go
index 130ce6a0bd..bbefb03dde 100644
--- a/storage/metric/memory.go
+++ b/storage/metric/memory.go
@@ -172,6 +172,8 @@ func (s *memorySeriesStorage) AppendSample(sample model.Sample) error {
 			labelNameValues = append(labelNameValues, fingerprint)
 			s.labelNameToFingerprints[k] = labelNameValues
 		}
+
+		s.Unlock()
 	}
 
 	series.add(sample.Timestamp, sample.Value)