diff --git a/os2.rkt b/os2.rkt
index e19f54a..da9876c 100644
--- a/os2.rkt
+++ b/os2.rkt
@@ -402,9 +402,31 @@
 				    (struct-copy process old-process
 						 [state new-state]))]))
 
+(define (preaction? a)
+  (or (add-role? a)
+      (delete-role? a)
+      (send-message? a)
+      (spawn? a)
+      (kill? a)))
+
+(define (action? a)
+  (or (preaction? a)
+      (and (at-meta-level? a)
+	   (preaction? (at-meta-level-preaction a)))))
+
+(define (valid-action? pid a)
+  (cond
+   [(action? a)]
+   [(eq? a #f) #f] ;; skip falses in action ConsTrees
+   [(void? a) #f] ;; skip voids in action ConsTrees
+   [else (log-warning (format "Illegal action ~v from pid ~v" a pid))
+	 #f]))
+
 (define (enqueue-actions state pid actions)
   (struct-copy vm state
-	       [pending-actions (append (reverse (for/list ([a (flatten actions)]) (cons pid a)))
+	       [pending-actions (append (reverse (for/list ([a (flatten actions)]
+							    #:when (valid-action? pid a))
+						   (cons pid a)))
 					(vm-pending-actions state))]))
 
 (define (((wrap-trapk pid trapk) . args) state)