diff --git a/lib/icinga/checkable-notification.cpp b/lib/icinga/checkable-notification.cpp
index 25218d8d3..4bcb63347 100644
--- a/lib/icinga/checkable-notification.cpp
+++ b/lib/icinga/checkable-notification.cpp
@@ -84,16 +84,19 @@ void Checkable::SendNotifications(NotificationType type, const CheckResult::Ptr&
 
 std::set<Notification::Ptr> Checkable::GetNotifications(void) const
 {
+	boost::mutex::scoped_lock lock(m_NotificationMutex);
 	return m_Notifications;
 }
 
 void Checkable::AddNotification(const Notification::Ptr& notification)
 {
+	boost::mutex::scoped_lock lock(m_NotificationMutex);
 	m_Notifications.insert(notification);
 }
 
 void Checkable::RemoveNotification(const Notification::Ptr& notification)
 {
+	boost::mutex::scoped_lock lock(m_NotificationMutex);
 	m_Notifications.erase(notification);
 }
 
diff --git a/lib/icinga/checkable.hpp b/lib/icinga/checkable.hpp
index f9f27450e..eb2e2dc90 100644
--- a/lib/icinga/checkable.hpp
+++ b/lib/icinga/checkable.hpp
@@ -302,6 +302,7 @@ private:
 
 	/* Notifications */
 	std::set<Notification::Ptr> m_Notifications;
+	mutable boost::mutex m_NotificationMutex;
 
 	/* Dependencies */
 	mutable boost::mutex m_DependencyMutex;