diff --git a/relaxng.c b/relaxng.c
index 5779e7f..345f354 100644
--- a/relaxng.c
+++ b/relaxng.c
@@ -507,7 +507,7 @@
  *
  * Handle a Relax NG Parsing error
  */
-static void
+static void LIBXML_ATTR_FORMAT(4,0)
 xmlRngPErr(xmlRelaxNGParserCtxtPtr ctxt, xmlNodePtr node, int error,
            const char *msg, const xmlChar * str1, const xmlChar * str2)
 {
@@ -541,7 +541,7 @@
  *
  * Handle a Relax NG Validation error
  */
-static void
+static void LIBXML_ATTR_FORMAT(4,0)
 xmlRngVErr(xmlRelaxNGValidCtxtPtr ctxt, xmlNodePtr node, int error,
            const char *msg, const xmlChar * str1, const xmlChar * str2)
 {