--- xglock/xglock.c
+++ xglock/xglock.c
@@ -666,7 +666,7 @@
     else if ( numberprocess == 0 ) {
         (void) execlp(XLOCK, XLOCK, "-parent", numberwidget,
                       MODE_STRING, LockProcs[mode].cmdline_arg, "-geometry", WINDOW_GEOMETRY, "-delay", "100000",
-                      "-nolock", "-inwindow", "+install", 0);
+                      "-nolock", "-inwindow", "+install", NULL);
     }
 }