diff --git a/collects/tests/racket/subprocess.rktl b/collects/tests/racket/subprocess.rktl
index c7369c4007..8e68fbd86b 100644
--- a/collects/tests/racket/subprocess.rktl
+++ b/collects/tests/racket/subprocess.rktl
@@ -410,6 +410,10 @@
   (parameterize ([current-subprocess-custodian-mode 'interrupt])
     (try (if (eq? 'windows (system-type)) 'running 'done-error) #t)))
 
+;; check that #f is an allowed mode:
+(parameterize ([current-subprocess-custodian-mode #f])
+  (test #f current-subprocess-custodian-mode))
+
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;;; process groups
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
diff --git a/src/racket/src/port.c b/src/racket/src/port.c
index 549bf3bc5e..79c6c634d2 100644
--- a/src/racket/src/port.c
+++ b/src/racket/src/port.c
@@ -8917,7 +8917,7 @@ static Scheme_Object *current_subproc_cust_mode (int argc, Scheme_Object *argv[]
 {
   return scheme_param_config("current-subprocess-custodian-mode", scheme_make_integer(MZCONFIG_SUBPROC_CUSTODIAN_MODE),
 			     argc, argv,
-			     -1, subproc_cust_mode_p, "'interrupt, 'kill, or #f", 0);
+			     -1, subproc_cust_mode_p, "'interrupt, 'kill, or #f", 1);
 }
 
 static Scheme_Object *subproc_group_on (int argc, Scheme_Object *argv[])