diff --git a/collects/net/base64-sig.ss b/collects/net/base64-sig.ss
index 485e63f90e..8440f8bbb1 100644
--- a/collects/net/base64-sig.ss
+++ b/collects/net/base64-sig.ss
@@ -5,7 +5,8 @@
   (provide net:base64^)
 
   (define-signature net:base64^
-    (base64-encode-stream
+    (base64-filename-safe
+     base64-encode-stream
      base64-decode-stream
      base64-encode
      base64-decode)))
diff --git a/collects/net/base64-unit.ss b/collects/net/base64-unit.ss
index 2bb1d075d6..f410facdd2 100644
--- a/collects/net/base64-unit.ss
+++ b/collects/net/base64-unit.ss
@@ -29,16 +29,22 @@
 	  (loop (add1 n))))
 
       (define digit-base64 (make-vector 64))
-      (let ([each-char (lambda (s e pos)
-			 (let loop ([i (char->integer s)][pos pos])
-			   (unless (> i (char->integer e))
-			     (vector-set! digit-base64 pos i)
-			     (loop (add1 i) (add1 pos)))))])
-	(each-char #\A #\Z 0)
-	(each-char #\a #\z 26)
-	(each-char #\0 #\9 52)
-	(each-char #\+ #\+ 62)
-	(each-char #\/ #\/ 63))
+      (define (each-char s e pos)
+        (let loop ([i (char->integer s)][pos pos])
+          (unless (> i (char->integer e))
+            (vector-set! digit-base64 pos i)
+            (loop (add1 i) (add1 pos)))))
+      (each-char #\A #\Z 0)
+      (each-char #\a #\z 26)
+      (each-char #\0 #\9 52)
+      (each-char #\+ #\+ 62)
+      (each-char #\/ #\/ 63)
+      
+      (define (base64-filename-safe)
+        (vector-set! base64-digit (char->integer #\-) 62)
+        (vector-set! base64-digit (char->integer #\_) 63)        
+        (each-char #\- #\- 62)
+        (each-char #\_ #\_ 63))
 
       (define (base64-decode-stream in out)
 	(let loop ([waiting 0][waiting-bits 0])