diff --git a/Misc/NEWS.d/next/Core and Builtins/2019-09-12-00-14-01.bpo-38124.n6E0H7.rst b/Misc/NEWS.d/next/Core and Builtins/2019-09-12-00-14-01.bpo-38124.n6E0H7.rst
new file mode 100644
index 00000000000..dca0ba5bc0c
--- /dev/null
+++ b/Misc/NEWS.d/next/Core and Builtins/2019-09-12-00-14-01.bpo-38124.n6E0H7.rst	
@@ -0,0 +1,2 @@
+Fix an off-by-one error in PyState_AddModule that could cause out-of-bounds
+memory access.
diff --git a/Python/pystate.c b/Python/pystate.c
index 8f30c94096b..f2924a8ff26 100644
--- a/Python/pystate.c
+++ b/Python/pystate.c
@@ -685,7 +685,7 @@ _PyState_AddModule(PyObject* module, struct PyModuleDef* def)
         if (!state->modules_by_index)
             return -1;
     }
-    while(PyList_GET_SIZE(state->modules_by_index) <= def->m_base.m_index)
+    while (PyList_GET_SIZE(state->modules_by_index) <= def->m_base.m_index)
         if (PyList_Append(state->modules_by_index, Py_None) < 0)
             return -1;
     Py_INCREF(module);
@@ -703,13 +703,11 @@ PyState_AddModule(PyObject* module, struct PyModuleDef* def)
         return -1;
     }
     index = def->m_base.m_index;
-    if (state->modules_by_index) {
-        if(PyList_GET_SIZE(state->modules_by_index) >= index) {
-            if(module == PyList_GET_ITEM(state->modules_by_index, index)) {
-                Py_FatalError("PyState_AddModule: Module already added!");
-                return -1;
-            }
-        }
+    if (state->modules_by_index &&
+        index < PyList_GET_SIZE(state->modules_by_index) &&
+        module == PyList_GET_ITEM(state->modules_by_index, index)) {
+        Py_FatalError("PyState_AddModule: Module already added!");
+        return -1;
     }
     return _PyState_AddModule(module, def);
 }