diff --git a/Modules/_collectionsmodule.c b/Modules/_collectionsmodule.c
--- a/Modules/_collectionsmodule.c
+++ b/Modules/_collectionsmodule.c
@@ -765,6 +765,14 @@
            Py_SIZE(deque) == 0);
 }
 
+static int
+valid_index(Py_ssize_t i, Py_ssize_t limit)
+{
+    /* Cast to size_t to make a single test
+     * checks whether 0 <= i < limit */
+    return (size_t) i < (size_t) limit;
+}
+
 static PyObject *
 deque_item(dequeobject *deque, Py_ssize_t i)
 {
@@ -772,7 +780,7 @@
     PyObject *item;
     Py_ssize_t n, index=i;
 
-    if ((size_t)i >= (size_t)Py_SIZE(deque)) {
+    if (!valid_index(i, Py_SIZE(deque))) {
         PyErr_SetString(PyExc_IndexError,
                         "deque index out of range");
         return NULL;
@@ -836,7 +844,7 @@
     block *b;
     Py_ssize_t n, len=Py_SIZE(deque), halflen=(len+1)>>1, index=i;
 
-    if ((size_t)i >= (size_t)len) {
+    if (!valid_index(i, len)) {
         PyErr_SetString(PyExc_IndexError,
                         "deque index out of range");
         return -1;