diff --git a/Include/internal/pycore_pathconfig.h b/Include/internal/pycore_pathconfig.h
index 395df498a0dc8f..00d7bbf23ea411 100644
--- a/Include/internal/pycore_pathconfig.h
+++ b/Include/internal/pycore_pathconfig.h
@@ -4,6 +4,10 @@
 extern "C" {
 #endif
 
+#ifndef Py_BUILD_CORE
+#  error "Py_BUILD_CORE must be defined to include this header"
+#endif
+
 PyAPI_FUNC(void) _Py_wstrlist_clear(
     int len,
     wchar_t **list);
diff --git a/setup.py b/setup.py
index 37c5dd58a6d2d7..a61330cbf83ed0 100644
--- a/setup.py
+++ b/setup.py
@@ -681,7 +681,8 @@ def detect_modules(self):
         exts.append( Extension("_json", ["_json.c"]) )
         # Python C API test module
         exts.append( Extension('_testcapi', ['_testcapimodule.c'],
-                               depends=['testcapi_long.h']) )
+                               depends=['testcapi_long.h'],
+                               define_macros=[('Py_BUILD_CORE', '')]) )
         # Python PEP-3118 (buffer protocol) test module
         exts.append( Extension('_testbuffer', ['_testbuffer.c']) )
         # Test loading multiple modules from one compiled file (http://bugs.python.org/issue16421)