diff --git a/src/lib/dhcp/packet_queue_ring.h b/src/lib/dhcp/packet_queue_ring.h
index 3477fff5c00be41c082e66f6d3dd7885a29e0a2b..315e2a0375721466b9f95d9b81ea040fb28161fb 100644
--- a/src/lib/dhcp/packet_queue_ring.h
+++ b/src/lib/dhcp/packet_queue_ring.h
@@ -105,6 +105,7 @@ public:
     /// @param to specifies the end of the queue to which the packet
     /// should be added.
     virtual void pushPacket(PacketTypePtr& packet, const QueueEnd& to=QueueEnd::BACK) {
+        isc::util::thread::Mutex::Locker lock(mutex_);
         if (to == QueueEnd::BACK) {
             queue_.push_back(packet);
         } else {