diff --git a/Modules/_collectionsmodule.c b/Modules/_collectionsmodule.c
index adbe7897004..8766d86dd3e 100644
--- a/Modules/_collectionsmodule.c
+++ b/Modules/_collectionsmodule.c
@@ -938,8 +938,7 @@ deque_reverse(dequeobject *deque, PyObject *unused)
     Py_ssize_t n = Py_SIZE(deque) >> 1;
     PyObject *tmp;
 
-    n++;
-    while (--n) {
+    while (--n >= 0) {
         /* Validate that pointers haven't met in the middle */
         assert(leftblock != rightblock || leftindex < rightindex);
         CHECK_NOT_END(leftblock);
@@ -981,8 +980,7 @@ deque_count(dequeobject *deque, PyObject *v)
     PyObject *item;
     int cmp;
 
-    n++;
-    while (--n) {
+    while (--n >= 0) {
         CHECK_NOT_END(b);
         item = b->data[index];
         cmp = PyObject_RichCompareBool(item, v, Py_EQ);
@@ -1019,8 +1017,7 @@ deque_contains(dequeobject *deque, PyObject *v)
     PyObject *item;
     int cmp;
 
-    n++;
-    while (--n) {
+    while (--n >= 0) {
         CHECK_NOT_END(b);
         item = b->data[index];
         cmp = PyObject_RichCompareBool(item, v, Py_EQ);
@@ -1088,13 +1085,13 @@ deque_index(dequeobject *deque, PyObject **args, Py_ssize_t nargs)
         }
     }
 
-    n = stop - i + 1;
-    while (--n) {
+    n = stop - i;
+    while (--n >= 0) {
         CHECK_NOT_END(b);
         item = b->data[index];
         cmp = PyObject_RichCompareBool(item, v, Py_EQ);
         if (cmp > 0)
-            return PyLong_FromSsize_t(stop - n);
+            return PyLong_FromSsize_t(stop - n - 1);
         if (cmp < 0)
             return NULL;
         if (start_state != deque->state) {
@@ -1228,16 +1225,14 @@ deque_item(dequeobject *deque, Py_ssize_t i)
         i = (Py_ssize_t)((size_t) i % BLOCKLEN);
         if (index < (Py_SIZE(deque) >> 1)) {
             b = deque->leftblock;
-            n++;
-            while (--n)
+            while (--n >= 0)
                 b = b->rightlink;
         } else {
             n = (Py_ssize_t)(
                     ((size_t)(deque->leftindex + Py_SIZE(deque) - 1))
                     / BLOCKLEN - n);
             b = deque->rightblock;
-            n++;
-            while (--n)
+            while (--n >= 0)
                 b = b->leftlink;
         }
     }
@@ -1281,16 +1276,14 @@ deque_ass_item(dequeobject *deque, Py_ssize_t i, PyObject *v)
     i = (Py_ssize_t)((size_t) i % BLOCKLEN);
     if (index <= halflen) {
         b = deque->leftblock;
-        n++;
-        while (--n)
+        while (--n >= 0)
             b = b->rightlink;
     } else {
         n = (Py_ssize_t)(
                 ((size_t)(deque->leftindex + Py_SIZE(deque) - 1))
                 / BLOCKLEN - n);
         b = deque->rightblock;
-        n++;
-        while (--n)
+        while (--n >= 0)
             b = b->leftlink;
     }
     Py_INCREF(v);