diff --git a/src/root-manager.scm b/src/root-manager.scm
index b0886aa..13f4db6 100644
--- a/src/root-manager.scm
+++ b/src/root-manager.scm
@@ -444,7 +444,7 @@
 (define (manager-parent root-wm manager)
   (let loop ((parent-window (window-parent (root-wm:dpy root-wm)
 					   (wm:window manager))))
-    (if (zero? parent-window)
+    (if (or (not parent-window) (zero? parent-window))
 	#f
 	(let ((l (filter (lambda (m)
 			   (equal? (wm:window m) parent-window))
diff --git a/src/utils.scm b/src/utils.scm
index 148f99f..b70e359 100644
--- a/src/utils.scm
+++ b/src/utils.scm
@@ -48,7 +48,7 @@
 		  (let ((res (fun (lambda () (sync-point-release sp)))))
 		    ;;(mdisplay "thread " id " returned: " res "\n")
 		    res)
-		  ))))
+		  ))) id)
       (sync-point-wait sp))))
 
 (define (with-lock lock thunk)
@@ -299,8 +299,11 @@
        (map-state is-viewable)))
 
 (define (window-mapped? dpy window)
-  (not (eq? (window-attribute:map-state (get-window-attributes dpy window))
-	    (map-state is-unmapped))))
+  (and window
+       (let ((attrs (get-window-attributes dpy window)))
+         (and attrs
+              (not (eq? (window-attribute:map-state attrs)
+                        (map-state is-unmapped)))))))
 
 (define (window-focused? dpy window)
   (eq? (get-input-focus-window dpy) window))
@@ -594,8 +597,9 @@
 			  (property:data p)
 			  '())))))
     (map (lambda (win)
-	   (window-attribute:colormap (get-window-attributes dpy win)))
-	 wins)))
+	   (window-attribute:colormap 
+            (get-window-attributes dpy win)))
+         (filter (lambda (x) x) wins))))
 
 (define (install-colormaps dpy window)
   (for-each (lambda (c)