diff --git a/server/message_cache.go b/server/message_cache.go
index e314ace3..a73650ad 100644
--- a/server/message_cache.go
+++ b/server/message_cache.go
@@ -8,6 +8,7 @@ import (
 	"net/netip"
 	"strings"
 	"time"
+	"sync"
 
 	_ "github.com/mattn/go-sqlite3" // SQLite driver
 	"heckel.io/ntfy/v2/log"
@@ -35,7 +36,7 @@ const (
 			priority INT NOT NULL,
 			tags TEXT NOT NULL,
 			click TEXT NOT NULL,
-			icon TEXT NOT NULL,			
+			icon TEXT NOT NULL,
 			actions TEXT NOT NULL,
 			attachment_name TEXT NOT NULL,
 			attachment_type TEXT NOT NULL,
@@ -72,30 +73,30 @@ const (
 	selectRowIDFromMessageID          = `SELECT id FROM messages WHERE mid = ?` // Do not include topic, see #336 and TestServer_PollSinceID_MultipleTopics
 	selectMessagesByIDQuery           = `
 		SELECT mid, time, expires, topic, message, title, priority, tags, click, icon, actions, attachment_name, attachment_type, attachment_size, attachment_expires, attachment_url, sender, user, content_type, encoding
-		FROM messages 
+		FROM messages
 		WHERE mid = ?
 	`
 	selectMessagesSinceTimeQuery = `
 		SELECT mid, time, expires, topic, message, title, priority, tags, click, icon, actions, attachment_name, attachment_type, attachment_size, attachment_expires, attachment_url, sender, user, content_type, encoding
-		FROM messages 
+		FROM messages
 		WHERE topic = ? AND time >= ? AND published = 1
 		ORDER BY time, id
 	`
 	selectMessagesSinceTimeIncludeScheduledQuery = `
 		SELECT mid, time, expires, topic, message, title, priority, tags, click, icon, actions, attachment_name, attachment_type, attachment_size, attachment_expires, attachment_url, sender, user, content_type, encoding
-		FROM messages 
+		FROM messages
 		WHERE topic = ? AND time >= ?
 		ORDER BY time, id
 	`
 	selectMessagesSinceIDQuery = `
 		SELECT mid, time, expires, topic, message, title, priority, tags, click, icon, actions, attachment_name, attachment_type, attachment_size, attachment_expires, attachment_url, sender, user, content_type, encoding
-		FROM messages 
+		FROM messages
 		WHERE topic = ? AND id > ? AND published = 1 
 		ORDER BY time, id
 	`
 	selectMessagesSinceIDIncludeScheduledQuery = `
 		SELECT mid, time, expires, topic, message, title, priority, tags, click, icon, actions, attachment_name, attachment_type, attachment_size, attachment_expires, attachment_url, sender, user, content_type, encoding
-		FROM messages 
+		FROM messages
 		WHERE topic = ? AND (id > ? OR published = 0)
 		ORDER BY time, id
 	`
@@ -105,10 +106,10 @@ const (
 		WHERE topic = ? AND published = 1
 		ORDER BY time DESC, id DESC
 		LIMIT 1
-  `
+	`
 	selectMessagesDueQuery = `
 		SELECT mid, time, expires, topic, message, title, priority, tags, click, icon, actions, attachment_name, attachment_type, attachment_size, attachment_expires, attachment_url, sender, user, content_type, encoding
-		FROM messages 
+		FROM messages
 		WHERE time <= ? AND published = 0
 		ORDER BY time, id
 	`
@@ -281,6 +282,7 @@ var (
 type messageCache struct {
 	db    *sql.DB
 	queue *util.BatchingQueue[*message]
+	mu    sync.Mutex
 	nop   bool
 }
 
@@ -340,6 +342,8 @@ func (c *messageCache) AddMessage(m *message) error {
 // addMessages synchronously stores a match of messages. If the database is locked, the transaction waits until
 // SQLite's busy_timeout is exceeded before erroring out.
 func (c *messageCache) addMessages(ms []*message) error {
+	c.mu.Lock()
+	defer c.mu.Unlock()
 	if c.nop {
 		return nil
 	}