diff --git a/README.md b/README.md
index d7538b6..b5dff03 100644
--- a/README.md
+++ b/README.md
@@ -1,7 +1,7 @@
 # Dependencies
 
 - [CHICKEN Scheme](https://call-cc.org) 5 or 6;
-- A few CHICKEN extensions, installable using `chicken-install -s html-parser openssl http-client matchable srfi-18`.
+- A few CHICKEN extensions, installable using `chicken-install -s format html-parser openssl http-client matchable srfi-18`.
 
 # Building
 
diff --git a/rain.scm b/rain.scm
index b3e8b7e..5cd9c8e 100644
--- a/rain.scm
+++ b/rain.scm
@@ -11,7 +11,7 @@
 
 (define (download-comic-image index)
   (format (current-error-port) "* downloading comic image ~a~%" index)
-  (call-with-output-file (format #f "rain-~a.png" index)
+  (call-with-output-file (format #f "rain-~4,48,d.png" (string->number index))
     (lambda (out)
       (call-with-input-request (find-comic-image index)
                                #f
diff --git a/rain.sld b/rain.sld
index 0c2c22e..ee0c24e 100755
--- a/rain.sld
+++ b/rain.sld
@@ -8,9 +8,9 @@ exec csi -s "$0" "$@"
    (scheme file)
    (chicken base)
    (chicken io)
-   (chicken format)
    (chicken port)
    (chicken process-context)
+   (format)
    (html-parser)
    (openssl)                            ; has to be before http-client
    (http-client)