diff --git a/src/calibre/gui2/main.py b/src/calibre/gui2/main.py
index f722cf226f..d43e618e9a 100644
--- a/src/calibre/gui2/main.py
+++ b/src/calibre/gui2/main.py
@@ -369,7 +369,7 @@ def build_pipe(print_error=True):
     t.start()
     t.join(3.0)
     if t.is_alive():
-        if iswindows():
+        if iswindows:
             cant_start()
         else:
             f = os.path.expanduser('~/.calibre_calibre GUI.lock')