diff -up dx-4.4.4/src/exec/libdx/xwindow.c.null dx-4.4.4/src/exec/libdx/xwindow.c
--- dx-4.4.4/src/exec/libdx/xwindow.c.null	2000-06-12 07:45:40.000000000 +0200
+++ dx-4.4.4/src/exec/libdx/xwindow.c	2009-11-08 15:10:13.000000000 +0100
@@ -29,7 +29,9 @@ Error
 DXRegisterWindowHandlerWithCheckProc(Error (*proc) (int, Pointer),
 		int (*check)(int, Pointer), Display *d, Pointer arg)
 {
-    int fd = ConnectionNumber(d);
+    int fd;
+    if (d) fd = ConnectionNumber(d);
+    else return ERROR;
 
     if (! DXRegisterInputHandlerWithCheckProc(proc, check, fd, arg))
 	return ERROR;