diff --git a/Doc/library/itertools.rst b/Doc/library/itertools.rst
index 530c29dec4..a5acf95707 100644
--- a/Doc/library/itertools.rst
+++ b/Doc/library/itertools.rst
@@ -408,7 +408,11 @@ loops that truncate the stream.
               self.tgtkey = self.currkey
               return (self.currkey, self._grouper(self.tgtkey, self.id))
           def _grouper(self, tgtkey, id):
-              while self.id is id and self.currkey == tgtkey:
+              while True:
+                  if self.id is not id:
+                      raise PyExc_RuntimeError('group changed during iteration')
+                  if self.currkey != tgtkey:
+                      break
                   yield self.currvalue
                   try:
                       self.currvalue = next(self.it)
diff --git a/Lib/test/test_itertools.py b/Lib/test/test_itertools.py
index 8353e68977..bd2b89b1e6 100644
--- a/Lib/test/test_itertools.py
+++ b/Lib/test/test_itertools.py
@@ -758,18 +758,18 @@ class TestBasicOps(unittest.TestCase):
         _, g1 = next(it)
         _, g2 = next(it)
         _, g3 = next(it)
-        self.assertEqual(list(g1), [])
-        self.assertEqual(list(g2), [])
-        self.assertEqual(next(g3), ('A', 5))
+        self.assertRaises(RuntimeError, next, g1)
+        self.assertRaises(RuntimeError, next, g2)
+        self.assertEqual(list(g3), s[5:])
         list(it)  # exhaust the groupby iterator
-        self.assertEqual(list(g3), [])
+        self.assertRaises(RuntimeError, next, g3)
 
         for proto in range(pickle.HIGHEST_PROTOCOL + 1):
             it = groupby(s, testR)
             _, g = next(it)
             next(it)
             next(it)
-            self.assertEqual(list(pickle.loads(pickle.dumps(g, proto))), [])
+            self.assertRaises(RuntimeError, pickle.dumps, g, proto)
 
         # Exercise pipes and filters style
         s = 'abracadabra'
diff --git a/Misc/NEWS.d/next/Library/2017-09-24-13-08-46.bpo-30346.Csse77.rst b/Misc/NEWS.d/next/Library/2017-09-24-13-42-37.bpo-30346.0w0qb9.rst
similarity index 90%
rename from Misc/NEWS.d/next/Library/2017-09-24-13-08-46.bpo-30346.Csse77.rst
rename to Misc/NEWS.d/next/Library/2017-09-24-13-42-37.bpo-30346.0w0qb9.rst
index 81ad0534fc..2da50f5577 100644
--- a/Misc/NEWS.d/next/Library/2017-09-24-13-08-46.bpo-30346.Csse77.rst
+++ b/Misc/NEWS.d/next/Library/2017-09-24-13-42-37.bpo-30346.0w0qb9.rst
@@ -1,2 +1,2 @@
-An iterator produced by itertools.groupby() iterator now becames exhausted
+An iterator produced by itertools.groupby() iterator now becames invalid
 after advancing the groupby iterator.
diff --git a/Modules/itertoolsmodule.c b/Modules/itertoolsmodule.c
index 2ac5ab24ec..b691918cea 100644
--- a/Modules/itertoolsmodule.c
+++ b/Modules/itertoolsmodule.c
@@ -287,8 +287,10 @@ _grouper_next(_grouperobject *igo)
     PyObject *newvalue, *newkey, *r;
     int rcmp;
 
-    if (gbo->currgrouper != igo)
+    if (gbo->currgrouper != igo) {
+        PyErr_SetString(PyExc_RuntimeError, "group changed during iteration");
         return NULL;
+    }
     if (gbo->currvalue == NULL) {
         newvalue = PyIter_Next(gbo->it);
         if (newvalue == NULL)
@@ -327,7 +329,8 @@ static PyObject *
 _grouper_reduce(_grouperobject *lz)
 {
     if (((groupbyobject *)lz->parent)->currgrouper != lz) {
-        return Py_BuildValue("N(())", _PyObject_GetBuiltin("iter"));
+        PyErr_SetString(PyExc_RuntimeError, "group changed during iteration");
+        return NULL;
     }
     return Py_BuildValue("O(OO)", Py_TYPE(lz), lz->parent, lz->tgtkey);
 }