diff --git a/lib/krb5.c b/lib/krb5.c
index b334ae2f40..4db19fb27f 100644
--- a/lib/krb5.c
+++ b/lib/krb5.c
@@ -434,6 +434,9 @@ static char level_to_char(int level)
 
 /* Send an FTP command defined by |message| and the optional arguments. The
    function returns the ftp_code. If an error occurs, -1 is returned. */
+static int ftp_send_command(struct Curl_easy *data, const char *message, ...)
+  CURL_PRINTF(2, 3);
+
 static int ftp_send_command(struct Curl_easy *data, const char *message, ...)
 {
   int ftp_code;