diff --git a/proxyd/proxyd/pkg/avg-sliding-window/sliding.go b/proxyd/proxyd/pkg/avg-sliding-window/sliding.go
index b0f2873..70c40be 100644
--- a/proxyd/proxyd/pkg/avg-sliding-window/sliding.go
+++ b/proxyd/proxyd/pkg/avg-sliding-window/sliding.go
@@ -1,6 +1,7 @@
 package avg_sliding_window
 
 import (
+	"sync"
 	"time"
 
 	lm "github.com/emirpasic/gods/maps/linkedhashmap"
@@ -44,6 +45,7 @@ type bucket struct {
 // Data points are rounded to nearest bucket of size `bucketSize`,
 // and evicted when they are too old based on `windowLength`
 type AvgSlidingWindow struct {
+	mux          sync.Mutex
 	bucketSize   time.Duration
 	windowLength time.Duration
 	clock        Clock
@@ -112,6 +114,9 @@ func (sw *AvgSlidingWindow) Incr() {
 func (sw *AvgSlidingWindow) AddWithTime(t time.Time, val float64) {
 	sw.advance()
 
+	defer sw.mux.Unlock()
+	sw.mux.Lock()
+
 	key := t.Round(sw.bucketSize)
 	if !sw.inWindow(key) {
 		return
@@ -139,6 +144,8 @@ func (sw *AvgSlidingWindow) AddWithTime(t time.Time, val float64) {
 
 // advance evicts old data points
 func (sw *AvgSlidingWindow) advance() {
+	defer sw.mux.Unlock()
+	sw.mux.Lock()
 	now := sw.clock.Now().Round(sw.bucketSize)
 	windowStart := now.Add(-sw.windowLength)
 	keys := sw.buckets.Keys()