diff --git a/Modules/_testcapi/parts.h b/Modules/_testcapi/parts.h
index c0e0f3aa1cc..a76ddd93c0e 100644
--- a/Modules/_testcapi/parts.h
+++ b/Modules/_testcapi/parts.h
@@ -1,5 +1,8 @@
 #include "Python.h"
 
+/* Always enable assertions */
+#undef NDEBUG
+
 int _PyTestCapi_Init_Vectorcall(PyObject *module);
 int _PyTestCapi_Init_VectorcallLimited(PyObject *module);
 int _PyTestCapi_Init_Heaptype(PyObject *module);