diff --git a/Lib/test/test_deque.py b/Lib/test/test_deque.py
--- a/Lib/test/test_deque.py
+++ b/Lib/test/test_deque.py
@@ -304,6 +304,20 @@
             s.insert(i, 'Z')
             self.assertEqual(list(d), s)
 
+    def test_index_bug_26194(self):
+        data = 'ABC'
+        for i in range(len(data) + 1):
+            d = deque(data, len(data))
+            d.insert(i, None)
+            s = list(data)
+            s.insert(i, None)
+            s.pop()
+            self.assertEqual(list(d), s)
+            if i < len(data):
+                self.assertIsNone(d[i])
+            else:
+                self.assertTrue(None not in d)
+
     def test_imul(self):
         for n in (-10, -1, 0, 1, 2, 10, 1000):
             d = deque()
diff --git a/Modules/_collectionsmodule.c b/Modules/_collectionsmodule.c
--- a/Modules/_collectionsmodule.c
+++ b/Modules/_collectionsmodule.c
@@ -1085,10 +1085,17 @@
     Py_ssize_t index;
     Py_ssize_t n = Py_SIZE(deque);
     PyObject *value;
+    PyObject *oldvalue;
     PyObject *rv;
 
     if (!PyArg_ParseTuple(args, "nO:insert", &index, &value))
         return NULL;
+    if (deque->maxlen == Py_SIZE(deque)) {
+        if (index >= deque->maxlen || Py_SIZE(deque) == 0)
+            Py_RETURN_NONE;
+        oldvalue = deque_pop(deque, NULL);
+        Py_DECREF(oldvalue);
+    }
     if (index >= n)
         return deque_append(deque, value);
     if (index <= -n || index == 0)