diff --git a/os2.rkt b/os2.rkt
index 4381849..2b1d342 100644
--- a/os2.rkt
+++ b/os2.rkt
@@ -251,45 +251,48 @@
     [(kill pid-to-kill reason) (do-kill (or pid-to-kill pid) reason state)]))
 
 (define (do-subscribe pid topic hs k state)
-  (define old-process (hash-ref (vm-processes state) pid))
-  (define eid-number (process-next-eid-number old-process))
-  (define new-eid (list pid eid-number))
-  (struct-copy vm (for*/fold ([state (run-trapk state pid k new-eid)])
-		      ([(matching-pid p) (in-hash (vm-processes state))]
-		       [matching-eid (in-set (process-endpoints p))]
-		       [e (in-value (hash-ref (vm-endpoints state) matching-eid))]
-		       [matching-topic (in-value (endpoint-topic e))]
-		       [flow-pattern (in-value (topic-intersection topic matching-topic))]
-		       #:when flow-pattern)
-		    (define inbound-flow (refine-topic matching-topic flow-pattern))
-		    (define outbound-flow (refine-topic topic flow-pattern))
-		    (let* ((state (if (flow-visible? topic inbound-flow)
-				      (run-trapk state
-						 pid
-						 (handlers-presence hs)
-						 new-eid
-						 inbound-flow)
-				      state))
-			   (state (if (flow-visible? matching-topic outbound-flow)
-				      (run-trapk state
-						 matching-pid
-						 (handlers-presence (endpoint-handlers e))
-						 matching-eid
-						 outbound-flow)
-				      state)))
-		      state))
-	       [processes (hash-set (vm-processes state)
-				    pid
-				    (struct-copy process old-process
-						 [next-eid-number (+ eid-number 1)]
-						 [endpoints
-						  (set-add (process-endpoints old-process)
-							   new-eid)]))]
-	       [endpoints (hash-set (vm-endpoints state)
-				    new-eid
-				    (endpoint new-eid
-					      topic
-					      hs))]))
+  (cond
+   [(hash-has-key? (vm-processes state) pid)
+    (define old-process (hash-ref (vm-processes state) pid))
+    (define eid-number (process-next-eid-number old-process))
+    (define new-eid (list pid eid-number))
+    (struct-copy vm (for*/fold ([state (run-trapk state pid k new-eid)])
+			([(matching-pid p) (in-hash (vm-processes state))]
+			 [matching-eid (in-set (process-endpoints p))]
+			 [e (in-value (hash-ref (vm-endpoints state) matching-eid))]
+			 [matching-topic (in-value (endpoint-topic e))]
+			 [flow-pattern (in-value (topic-intersection topic matching-topic))]
+			 #:when flow-pattern)
+		      (define inbound-flow (refine-topic matching-topic flow-pattern))
+		      (define outbound-flow (refine-topic topic flow-pattern))
+		      (let* ((state (if (flow-visible? topic inbound-flow)
+					(run-trapk state
+						   pid
+						   (handlers-presence hs)
+						   new-eid
+						   inbound-flow)
+					state))
+			     (state (if (flow-visible? matching-topic outbound-flow)
+					(run-trapk state
+						   matching-pid
+						   (handlers-presence (endpoint-handlers e))
+						   matching-eid
+						   outbound-flow)
+					state)))
+			state))
+		 [processes (hash-set (vm-processes state)
+				      pid
+				      (struct-copy process old-process
+						   [next-eid-number (+ eid-number 1)]
+						   [endpoints
+						    (set-add (process-endpoints old-process)
+							     new-eid)]))]
+		 [endpoints (hash-set (vm-endpoints state)
+				      new-eid
+				      (endpoint new-eid
+						topic
+						hs))])]
+   [else state]))
 
 (define (do-unsubscribe pid eid reason state)
   (cond