diff --git a/lib/base64.h b/lib/base64.h
index f731acfadeaf84b79697c764bddcc0c52bb32532..307148eef9dceff02358d1111d397981c34c5337 100644
--- a/lib/base64.h
+++ b/lib/base64.h
@@ -23,5 +23,5 @@
  * $Id$
  ***************************************************************************/
 size_t Curl_base64_encode(const char *input, size_t size, char **str);
-size_t Curl_base64_decode(char *dest, const char *source);
+size_t Curl_base64_decode(const char *source, char *dest);
 #endif