diff --git a/metrics/generic/generic.go b/metrics/generic/generic.go
index 7e68252..9fc6e01 100644
--- a/metrics/generic/generic.go
+++ b/metrics/generic/generic.go
@@ -208,7 +208,7 @@
 // ApproximateMovingAverage returns the approximate moving average of observations.
 func (h *SimpleHistogram) ApproximateMovingAverage() float64 {
 	h.mtx.RLock()
-	h.mtx.RUnlock()
+	defer h.mtx.RUnlock()
 	return h.avg
 }