diff --git a/valid.c b/valid.c
index bf65303..9fa6c14 100644
--- a/valid.c
+++ b/valid.c
@@ -2880,6 +2880,13 @@
  */
 int
 xmlIsRef(xmlDocPtr doc, xmlNodePtr elem, xmlAttrPtr attr) {
+    if (attr == NULL)
+        return(0);
+    if (doc == NULL) {
+        doc = attr->doc;
+	if (doc == NULL) return(0);
+    }
+
     if ((doc->intSubset == NULL) && (doc->extSubset == NULL)) {
         return(0);
     } else if (doc->type == XML_HTML_DOCUMENT_NODE) {
@@ -2888,6 +2895,7 @@
     } else {
         xmlAttributePtr attrDecl;
 
+        if (elem == NULL) return(0);
         attrDecl = xmlGetDtdAttrDesc(doc->intSubset, elem->name, attr->name);
         if ((attrDecl == NULL) && (doc->extSubset != NULL))
             attrDecl = xmlGetDtdAttrDesc(doc->extSubset,