diff --git a/util.go b/util.go
index a6264c6..f15927a 100644
--- a/util.go
+++ b/util.go
@@ -1,8 +1,13 @@
 package posthog
 
+import (
+	"sync"
+)
+
 type SizeLimitedMap struct {
 	ids  map[string][]string
 	size int
+	mu   sync.Mutex
 }
 
 func newSizeLimitedMap(size int) *SizeLimitedMap {
@@ -15,6 +20,8 @@ func newSizeLimitedMap(size int) *SizeLimitedMap {
 }
 
 func (sizeLimitedMap *SizeLimitedMap) add(key string, element string) {
+	sizeLimitedMap.mu.Lock()
+	defer sizeLimitedMap.mu.Unlock()
 
 	if len(sizeLimitedMap.ids) >= sizeLimitedMap.size {
 		sizeLimitedMap.ids = map[string][]string{}
@@ -28,6 +35,9 @@ func (sizeLimitedMap *SizeLimitedMap) add(key string, element string) {
 }
 
 func (sizeLimitedMap *SizeLimitedMap) contains(key string, element string) bool {
+	sizeLimitedMap.mu.Lock()
+	defer sizeLimitedMap.mu.Unlock()
+
 	if val, ok := sizeLimitedMap.ids[key]; ok {
 		for _, v := range val {
 			if v == element {
@@ -40,5 +50,8 @@ func (sizeLimitedMap *SizeLimitedMap) contains(key string, element string) bool
 }
 
 func (sizeLimitedMap *SizeLimitedMap) count() int {
+	sizeLimitedMap.mu.Lock()
+	defer sizeLimitedMap.mu.Unlock()
+
 	return len(sizeLimitedMap.ids)
 }