diff --git a/expat/lib/xmlparse.c b/expat/lib/xmlparse.c
index 3dbeff90..4b5e38ba 100644
--- a/expat/lib/xmlparse.c
+++ b/expat/lib/xmlparse.c
@@ -1648,7 +1648,7 @@ XML_SetHashSalt(XML_Parser parser,
 enum XML_Status XMLCALL
 XML_Parse(XML_Parser parser, const char *s, int len, int isFinal)
 {
-  if ((parser == NULL) || ((s == NULL) && (len != 0))) {
+  if ((parser == NULL) || (len < 0) || ((s == NULL) && (len != 0))) {
     errorCode = XML_ERROR_INVALID_ARGUMENT;
     return XML_STATUS_ERROR;
   }