diff --git a/lib/chibi/process.scm b/lib/chibi/process.scm
index b4221824..4e332f82 100644
--- a/lib/chibi/process.scm
+++ b/lib/chibi/process.scm
@@ -1,4 +1,12 @@
 
+(cond-expand
+ (plan9
+  (define (exit . o)
+    (%exit (if (pair? o) (if (string? (car o)) (car o) "error") ""))))
+ (else
+  (define (exit . o)
+    (%exit (if (pair? o) (if (integer? (car o)) (car o) 1) 0)))))
+
 (cond-expand
  (bsd
   (define (process-command-line pid)
diff --git a/lib/chibi/process.stub b/lib/chibi/process.stub
index fca6db84..d37e1b84 100644
--- a/lib/chibi/process.stub
+++ b/lib/chibi/process.stub
@@ -117,7 +117,9 @@
 
 ;;> Exits the current process immediately.  Finalizers are not run.
 
-(define-c void exit (int))
+(cond-expand
+ (plan9 (define-c void (%exit exit) (string)))
+ (else (define-c void (%exit exit) (int))))
 
 ;;> Replace the current process with the given command.  Finalizers
 ;;> are not run.