diff --git a/hmac.scm b/hmac.scm
index 2f7b7d8..bb82bc6 100644
--- a/hmac.scm
+++ b/hmac.scm
@@ -9,21 +9,22 @@
 (use message-digest-port)
 
 (define (hmac key digest-primitive #!optional (block-size 64))
-  (when (> (string-length key) block-size)
-    (set! key (call-with-output-digest digest-primitive (cut display key <>) 'string)))
-  (set! key (string-pad-right key block-size (integer->char 0)))
-  (let ((ipad (string-map (lambda (c) (integer->char (bitwise-xor (char->integer c) #x36))) key))
-	(opad (string-map (lambda (c) (integer->char (bitwise-xor (char->integer c) #x5c))) key)))
-    (lambda (message)
-      (call-with-output-digest
-        digest-primitive
-        (cut display
-             (string-append opad
-                            (call-with-output-digest
+  (let ((key_ key))
+    (when (> (string-length key_) block-size)
+          (set! key_ (call-with-output-digest digest-primitive (cut display key_ <>) 'string)))
+    (set! key_ (string-pad-right key_ block-size (integer->char 0)))
+    (let ((ipad (string-map (lambda (c) (integer->char (bitwise-xor (char->integer c) #x36))) key_))
+          (opad (string-map (lambda (c) (integer->char (bitwise-xor (char->integer c) #x5c))) key_)))
+      (lambda (message)
+        (call-with-output-digest
+         digest-primitive
+         (cut display
+              (string-append opad
+                             (call-with-output-digest
                               digest-primitive
                               (cut display (string-append ipad message) <>)
                               'string))
-             <>)
-        'string))))
+              <>)
+         'string)))))
 
 )