diff --git a/src/singleinstance.py b/src/singleinstance.py
index d6337ff6..8a7fb68e 100644
--- a/src/singleinstance.py
+++ b/src/singleinstance.py
@@ -55,6 +55,7 @@ class singleinstance:
             else:
                 pidLine = "%i\n" % self.lockPid
                 self.fd.write(pidLine)
+                self.fd.flush()
         else:  # non Windows
             self.fp = open(self.lockfile, 'w')
             try: