diff --git a/lib/ftp.h b/lib/ftp.h
index ae923cdfe56a775125e040e7101c2138a2a16f1d..bce339563903761d4d778ae75c75f57af448e5e7 100644
--- a/lib/ftp.h
+++ b/lib/ftp.h
@@ -28,7 +28,7 @@ CURLcode Curl_ftp_done(struct connectdata *conn);
 CURLcode Curl_ftp_connect(struct connectdata *conn);
 CURLcode Curl_ftp_disconnect(struct connectdata *conn);
 
-size_t Curl_ftpsendf(int fd, struct connectdata *, char *fmt, ...);
+size_t Curl_ftpsendf(int fd, struct connectdata *, const char *fmt, ...);
 
 /* The kerberos stuff needs this: */
 int Curl_GetFTPResponse(int sockfd, char *buf,