diff --git a/lib/matplotlib/backends/backend_qt5.py b/lib/matplotlib/backends/backend_qt5.py
index b36e6564287b..181dcb4843f7 100644
--- a/lib/matplotlib/backends/backend_qt5.py
+++ b/lib/matplotlib/backends/backend_qt5.py
@@ -1091,9 +1091,11 @@ def trigger_manager_draw(manager):
     def mainloop():
         old_signal = signal.getsignal(signal.SIGINT)
         # allow SIGINT exceptions to close the plot window.
-        signal.signal(signal.SIGINT, signal.SIG_DFL)
+        if old_signal:
+            signal.signal(signal.SIGINT, signal.SIG_DFL)
         try:
             qApp.exec_()
         finally:
             # reset the SIGINT exception handler
-            signal.signal(signal.SIGINT, old_signal)
+            if old_signal:
+                signal.signal(signal.SIGINT, old_signal)