diff --git a/immutables/_map.c b/immutables/_map.c
index 167385a1..9f0a5866 100644
--- a/immutables/_map.c
+++ b/immutables/_map.c
@@ -1,4 +1,5 @@
 #include <stddef.h> /* For offsetof */
+#include "pythoncapi_compat.h"
 #include "_map.h"
 
 
@@ -571,7 +572,7 @@ map_node_bitmap_new(Py_ssize_t size, uint64_t mutid)
         return NULL;
     }
 
-    Py_SIZE(node) = size;
+    Py_SET_SIZE(node, size);
 
     for (i = 0; i < size; i++) {
         node->b_array[i] = NULL;
@@ -1356,7 +1357,7 @@ map_node_collision_new(int32_t hash, Py_ssize_t size, uint64_t mutid)
         node->c_array[i] = NULL;
     }
 
-    Py_SIZE(node) = size;
+    Py_SET_SIZE(node, size);
     node->c_hash = hash;
 
     node->c_mutid = mutid;
diff --git a/immutables/pythoncapi_compat.h b/immutables/pythoncapi_compat.h
new file mode 100644
index 00000000..5c661ab9
--- /dev/null
+++ b/immutables/pythoncapi_compat.h
@@ -0,0 +1,278 @@
+// Header file providing new functions of the Python C API to old Python
+// versions.
+//
+// File distributed under the MIT license.
+//
+// Homepage:
+// https://github.com/pythoncapi/pythoncapi_compat
+//
+// Latest version:
+// https://raw.githubusercontent.com/pythoncapi/pythoncapi_compat/master/pythoncapi_compat.h
+
+#ifndef PYTHONCAPI_COMPAT
+#define PYTHONCAPI_COMPAT
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#include <Python.h>
+#include "frameobject.h"          // PyFrameObject, PyFrame_GetBack()
+
+
+// Cast argument to PyObject* type.
+#ifndef _PyObject_CAST
+#  define _PyObject_CAST(op) ((PyObject*)(op))
+#endif
+
+
+// bpo-42262 added Py_NewRef() to Python 3.10.0a3
+#if PY_VERSION_HEX < 0x030a00A3 && !defined(Py_NewRef)
+static inline PyObject* _Py_NewRef(PyObject *obj)
+{
+    Py_INCREF(obj);
+    return obj;
+}
+#define Py_NewRef(obj) _Py_NewRef(_PyObject_CAST(obj))
+#endif
+
+
+// bpo-42262 added Py_XNewRef() to Python 3.10.0a3
+#if PY_VERSION_HEX < 0x030a00A3 && !defined(Py_XNewRef)
+static inline PyObject* _Py_XNewRef(PyObject *obj)
+{
+    Py_XINCREF(obj);
+    return obj;
+}
+#define Py_XNewRef(obj) _Py_XNewRef(_PyObject_CAST(obj))
+#endif
+
+
+// bpo-39573 added Py_SET_REFCNT() to Python 3.9.0a4
+#if PY_VERSION_HEX < 0x030900A4 && !defined(Py_SET_REFCNT)
+static inline void _Py_SET_REFCNT(PyObject *ob, Py_ssize_t refcnt)
+{
+    ob->ob_refcnt = refcnt;
+}
+#define Py_SET_REFCNT(ob, refcnt) _Py_SET_REFCNT((PyObject*)(ob), refcnt)
+#endif
+
+
+// bpo-39573 added Py_SET_TYPE() to Python 3.9.0a4
+#if PY_VERSION_HEX < 0x030900A4 && !defined(Py_SET_TYPE)
+static inline void
+_Py_SET_TYPE(PyObject *ob, PyTypeObject *type)
+{
+    ob->ob_type = type;
+}
+#define Py_SET_TYPE(ob, type) _Py_SET_TYPE((PyObject*)(ob), type)
+#endif
+
+
+// bpo-39573 added Py_SET_SIZE() to Python 3.9.0a4
+#if PY_VERSION_HEX < 0x030900A4 && !defined(Py_SET_SIZE)
+static inline void
+_Py_SET_SIZE(PyVarObject *ob, Py_ssize_t size)
+{
+    ob->ob_size = size;
+}
+#define Py_SET_SIZE(ob, size) _Py_SET_SIZE((PyVarObject*)(ob), size)
+#endif
+
+
+// bpo-40421 added PyFrame_GetCode() to Python 3.9.0b1
+#if PY_VERSION_HEX < 0x030900B1
+static inline PyCodeObject*
+PyFrame_GetCode(PyFrameObject *frame)
+{
+    PyCodeObject *code;
+    assert(frame != NULL);
+    code = frame->f_code;
+    assert(code != NULL);
+    Py_INCREF(code);
+    return code;
+}
+#endif
+
+static inline PyCodeObject*
+_PyFrame_GetCodeBorrow(PyFrameObject *frame)
+{
+    PyCodeObject *code = PyFrame_GetCode(frame);
+    Py_DECREF(code);
+    return code;  // borrowed reference
+}
+
+
+// bpo-40421 added PyFrame_GetCode() to Python 3.9.0b1
+#if PY_VERSION_HEX < 0x030900B1
+static inline PyFrameObject*
+PyFrame_GetBack(PyFrameObject *frame)
+{
+    PyFrameObject *back;
+    assert(frame != NULL);
+    back = frame->f_back;
+    Py_XINCREF(back);
+    return back;
+}
+#endif
+
+static inline PyFrameObject*
+_PyFrame_GetBackBorrow(PyFrameObject *frame)
+{
+    PyFrameObject *back = PyFrame_GetBack(frame);
+    Py_XDECREF(back);
+    return back;  // borrowed reference
+}
+
+
+// bpo-39947 added PyThreadState_GetInterpreter() to Python 3.9.0a5
+#if PY_VERSION_HEX < 0x030900A5
+static inline PyInterpreterState *
+PyThreadState_GetInterpreter(PyThreadState *tstate)
+{
+    assert(tstate != NULL);
+    return tstate->interp;
+}
+#endif
+
+
+// bpo-40429 added PyThreadState_GetFrame() to Python 3.9.0b1
+#if PY_VERSION_HEX < 0x030900B1
+static inline PyFrameObject*
+PyThreadState_GetFrame(PyThreadState *tstate)
+{
+    PyFrameObject *frame;
+    assert(tstate != NULL);
+    frame = tstate->frame;
+    Py_XINCREF(frame);
+    return frame;
+}
+#endif
+
+static inline PyFrameObject*
+_PyThreadState_GetFrameBorrow(PyThreadState *tstate)
+{
+    PyFrameObject *frame = PyThreadState_GetFrame(tstate);
+    Py_XDECREF(frame);
+    return frame;  // borrowed reference
+}
+
+
+// bpo-39947 added PyInterpreterState_Get() to Python 3.9.0a5
+#if PY_VERSION_HEX < 0x030900A5
+static inline PyInterpreterState *
+PyInterpreterState_Get(void)
+{
+    PyThreadState *tstate;
+    PyInterpreterState *interp;
+
+    tstate = PyThreadState_GET();
+    if (tstate == NULL) {
+        Py_FatalError("GIL released (tstate is NULL)");
+    }
+    interp = tstate->interp;
+    if (interp == NULL) {
+        Py_FatalError("no current interpreter");
+    }
+    return interp;
+}
+#endif
+
+
+// bpo-39947 added PyInterpreterState_Get() to Python 3.9.0a6
+#if 0x030700A1 <= PY_VERSION_HEX && PY_VERSION_HEX < 0x030900A6
+static inline uint64_t
+PyThreadState_GetID(PyThreadState *tstate)
+{
+    assert(tstate != NULL);
+    return tstate->id;
+}
+#endif
+
+
+// bpo-37194 added PyObject_CallNoArgs() to Python 3.9.0a1
+#if PY_VERSION_HEX < 0x030900A1
+static inline PyObject*
+PyObject_CallNoArgs(PyObject *func)
+{
+    return PyObject_CallFunctionObjArgs(func, NULL);
+}
+#endif
+
+
+// bpo-39245 made PyObject_CallOneArg() public (previously called
+// _PyObject_CallOneArg) in Python 3.9.0a4
+#if PY_VERSION_HEX < 0x030900A4
+static inline PyObject*
+PyObject_CallOneArg(PyObject *func, PyObject *arg)
+{
+    return PyObject_CallFunctionObjArgs(func, arg, NULL);
+}
+#endif
+
+
+// bpo-40024 added PyModule_AddType() to Python 3.9.0a5
+#if PY_VERSION_HEX < 0x030900A5
+static inline int
+PyModule_AddType(PyObject *module, PyTypeObject *type)
+{
+    const char *name, *dot;
+
+    if (PyType_Ready(type) < 0) {
+        return -1;
+    }
+
+    // inline _PyType_Name()
+    name = type->tp_name;
+    assert(name != NULL);
+    dot = strrchr(name, '.');
+    if (dot != NULL) {
+        name = dot + 1;
+    }
+
+    Py_INCREF(type);
+    if (PyModule_AddObject(module, name, (PyObject *)type) < 0) {
+        Py_DECREF(type);
+        return -1;
+    }
+
+    return 0;
+}
+#endif
+
+
+// bpo-40241 added PyObject_GC_IsTracked() to Python 3.9.0a6.
+// bpo-4688 added _PyObject_GC_IS_TRACKED() to Python 2.7.0a2.
+#if PY_VERSION_HEX < 0x030900A6
+static inline int
+PyObject_GC_IsTracked(PyObject* obj)
+{
+    return (PyObject_IS_GC(obj) && _PyObject_GC_IS_TRACKED(obj));
+}
+#endif
+
+// bpo-40241 added PyObject_GC_IsFinalized() to Python 3.9.0a6.
+// bpo-18112 added _PyGCHead_FINALIZED() to Python 3.4.0 final.
+#if PY_VERSION_HEX < 0x030900A6 && PY_VERSION_HEX >= 0x030400F0
+static inline int
+PyObject_GC_IsFinalized(PyObject *obj)
+{
+    return (PyObject_IS_GC(obj) && _PyGCHead_FINALIZED((PyGC_Head *)(obj)-1));
+}
+#endif
+
+
+// bpo-39573 added Py_IS_TYPE() to Python 3.9.0a4
+#if PY_VERSION_HEX < 0x030900A4 && !defined(Py_IS_TYPE)
+static inline int
+_Py_IS_TYPE(const PyObject *ob, const PyTypeObject *type) {
+    return ob->ob_type == type;
+}
+#define Py_IS_TYPE(ob, type) _Py_IS_TYPE((const PyObject*)(ob), type)
+#endif
+
+
+#ifdef __cplusplus
+}
+#endif
+#endif  // PYTHONCAPI_COMPAT