diff --git a/Lib/test/test_pyexpat.py b/Lib/test/test_pyexpat.py
index de5cded6c5a..649d5d23bfa 100644
--- a/Lib/test/test_pyexpat.py
+++ b/Lib/test/test_pyexpat.py
@@ -559,6 +559,24 @@ class ChardataBufferTest(unittest.TestCase):
         parser.Parse(xml2, 1)
         self.assertEquals(self.n, 4)
 
+class MalformedInputText(unittest.TestCase):
+    def test1(self):
+        xml = "\0\r\n"
+        parser = expat.ParserCreate()
+        try:
+            parser.Parse(xml, True)
+            self.fail()
+        except expat.ExpatError as e:
+            self.assertEquals(str(e), 'no element found: line 2, column 1')
+
+    def test2(self):
+        xml = "<?xml version\xc2\x85='1.0'?>\r\n"
+        parser = expat.ParserCreate()
+        try:
+            parser.Parse(xml, True)
+            self.fail()
+        except expat.ExpatError as e:
+            self.assertEquals(str(e), 'XML declaration not well-formed: line 1, column 14')
 
 def test_main():
     run_unittest(SetAttributeTest,
@@ -569,7 +587,8 @@ def test_main():
                  HandlerExceptionTest,
                  PositionTest,
                  sf1296433Test,
-                 ChardataBufferTest)
+                 ChardataBufferTest,
+                 MalformedInputText)
 
 if __name__ == "__main__":
     test_main()
diff --git a/Misc/ACKS b/Misc/ACKS
index defa80273ec..dc87dc22c66 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -183,6 +183,7 @@ Ismail Donmez
 Dima Dorfman
 Cesar Douady
 Dean Draayer
+Fred L. Drake, Jr.
 John DuBois
 Paul Dubois
 Graham Dumpleton
@@ -371,7 +372,6 @@ Irmen de Jong
 Lucas de Jonge
 John Jorgensen
 Jens B. Jorgensen
-Fred L. Drake, Jr.
 Andreas Jung
 Tattoo Mabonzo K.
 Bob Kahn
@@ -408,6 +408,7 @@ Holger Krekel
 Michael Kremer
 Fabian Kreutz
 Hannu Krosing
+Ivan Krstić
 Andrew Kuchling
 Vladimir Kushnir
 Cameron Laird
diff --git a/Misc/NEWS b/Misc/NEWS
index 54758ae146f..42e14a2acf8 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -1227,6 +1227,9 @@ C-API
 Extension Modules
 -----------------
 
+- Fix a segfault in expat when given a specially crafted input lead to the
+  tokenizer not stopping.
+
 - Issue #6561: '\d' in a regex now matches only characters with
   Unicode category 'Nd' (Number, Decimal Digit).  Previously it also
   matched characters with category 'No'.
diff --git a/Modules/expat/xmltok_impl.c b/Modules/expat/xmltok_impl.c
index 0ee57abb1f4..f793a6b8c76 100644
--- a/Modules/expat/xmltok_impl.c
+++ b/Modules/expat/xmltok_impl.c
@@ -1741,7 +1741,7 @@ PREFIX(updatePosition)(const ENCODING *enc,
                        const char *end,
                        POSITION *pos)
 {
-  while (ptr != end) {
+  while (ptr < end) {
     switch (BYTE_TYPE(enc, ptr)) {
 #define LEAD_CASE(n) \
     case BT_LEAD ## n: \