--- /usr/share/common-lisp/source/acl-compat/sbcl/acl-mp.lisp	2005-08-08 10:16:33.000000000 +0200
+++ acl-mp.lisp	2005-09-01 20:28:45.000000000 +0200
@@ -290,4 +290,5 @@
     (decf ,place)))
 
 (defun process-active-p (process)
-  (sb-thread:thread-alive-p (process-id process)))
+  (and (process-id process)
+       (sb-thread:thread-alive-p (process-id process))))