diff --git a/collects/mzlib/tar.ss b/collects/mzlib/tar.ss
index 2af5586..d5f6cda 100644
--- a/collects/mzlib/tar.ss
+++ b/collects/mzlib/tar.ss
@@ -1,5 +1,5 @@
 (module tar mzscheme
-  (require (lib "deflate.ss") (lib "file.ss"))
+  (require (lib "deflate.ss") (lib "file.ss") (lib "etc.ss"))
 
   (define tar-block-size 512)
   (define tar-name-length 100)
@@ -119,13 +119,13 @@
   ;; tar-write : (listof relative-path) ->
   ;; writes a tar file to current-output-port
   (provide tar->output)
-  (define (tar->output files . out)
-    (parameterize ([current-output-port
-                    (if (pair? out) (car out) (current-output-port))])
-      (let* ([buf (new-block)] [entry (tar-one-entry buf)])
-        (for-each entry files)
-        ;; two null blocks end-marker
-        (write-bytes buf) (write-bytes buf))))
+  (define tar->output 
+    (opt-lambda (files [out (current-output-port)])
+      (parameterize ([current-output-port out])
+        (let* ([buf (new-block)] [entry (tar-one-entry buf)])
+          (for-each entry files)
+          ;; two null blocks end-marker
+          (write-bytes buf) (write-bytes buf)))))
 
   ;; tar : output-file paths ->
   (provide tar)
diff --git a/collects/mzlib/zip.ss b/collects/mzlib/zip.ss
index 1dbfd6a..e476eb6 100644
--- a/collects/mzlib/zip.ss
+++ b/collects/mzlib/zip.ss
@@ -1,7 +1,7 @@
 ;; A modification of Dave Herman's zip module
 
 (module zip mzscheme
-  (require (lib "deflate.ss") (lib "file.ss"))
+  (require (lib "deflate.ss") (lib "file.ss") (lib "etc.ss"))
 
   ;; ===========================================================================
   ;; DATA DEFINITIONS
@@ -246,19 +246,19 @@
   ;; zip-write : (listof relative-path) ->
   ;; writes a zip file to current-output-port
   (provide zip->output)
-  (define (zip->output files . out)
-    (parameterize ([current-output-port
-                    (if (pair? out) (car out) (current-output-port))])
-      (let* ([seekable? (seekable-port? (current-output-port))]
-             [headers ; note: MzScheme's `map' is always left-to-right
-              (map (lambda (file)
-                     (zip-one-entry (build-metadata file) seekable?))
-                   files)])
+  (define zip->output 
+    (opt-lambda (files [out (current-output-port)])
+      (parameterize ([current-output-port out])
+        (let* ([seekable? (seekable-port? (current-output-port))]
+               [headers ; note: MzScheme's `map' is always left-to-right
+                (map (lambda (file)
+                       (zip-one-entry (build-metadata file) seekable?))
+                     files)])
+          (when (zip-verbose)
+            (fprintf (current-error-port) "zip: writing headers...\n"))
+          (write-central-directory headers))
         (when (zip-verbose)
-          (fprintf (current-error-port) "zip: writing headers...\n"))
-        (write-central-directory headers))
-      (when (zip-verbose)
-        (fprintf (current-error-port) "zip: done.\n"))))
+          (fprintf (current-error-port) "zip: done.\n")))))
 
   ;; zip : output-file paths ->
   (provide zip)