diff --git a/WebHostLib/autolauncher.py b/WebHostLib/autolauncher.py
index 97db4ca6..dc88c202 100644
--- a/WebHostLib/autolauncher.py
+++ b/WebHostLib/autolauncher.py
@@ -53,7 +53,7 @@ else:  # unix
         def __enter__(self):
             try:
                 self.fp = open(self.lockfile, "wb")
-                fcntl.flock(self.fp.fileno(), fcntl.LOCK_EX)
+                fcntl.flock(self.fp.fileno(), fcntl.LOCK_EX | fcntl.LOCK_NB)
             except OSError as e:
                 raise AlreadyRunningException() from e