diff --git a/src/asyncrpcqueue.cpp b/src/asyncrpcqueue.cpp
index 4994337c7..a6f35d367 100644
--- a/src/asyncrpcqueue.cpp
+++ b/src/asyncrpcqueue.cpp
@@ -183,6 +183,7 @@ void AsyncRPCQueue::addWorker() {
  * Return the number of worker threads spawned by the queue
  */
 size_t AsyncRPCQueue::getNumberOfWorkers() const {
+    std::lock_guard<std::mutex> guard(lock_);
     return workers_.size();
 }