diff --git a/src/gui/gui.cpp b/src/gui/gui.cpp
index d2d8a9e10..e5aa78e1d 100644
--- a/src/gui/gui.cpp
+++ b/src/gui/gui.cpp
@@ -3550,7 +3550,7 @@ bool FurnaceGUI::loop() {
     // backup trigger
     if (modified) {
       if (backupTimer>0) {
-        backupTimer-=ImGui::GetIO().DeltaTime;
+        backupTimer=(backupTimer-ImGui::GetIO().DeltaTime);
         if (backupTimer<=0) {
           backupTask=std::async(std::launch::async,[this]() -> bool {
             if (backupPath==curFileName) {
diff --git a/src/gui/gui.h b/src/gui/gui.h
index b976a7cb7..6f93c9f9a 100644
--- a/src/gui/gui.h
+++ b/src/gui/gui.h
@@ -799,7 +799,7 @@ class FurnaceGUI {
   double aboutScroll, aboutSin;
   float aboutHue;
 
-  double backupTimer;
+  std::atomic<double> backupTimer;
   std::future<bool> backupTask;
   std::mutex backupLock;
   String backupPath;