diff --git a/Modules/_testcapimodule.c b/Modules/_testcapimodule.c
index 8e18b14a0a8..101d54932d9 100644
--- a/Modules/_testcapimodule.c
+++ b/Modules/_testcapimodule.c
@@ -10,7 +10,10 @@
 
    The Visual Studio projects builds _testcapi with Py_BUILD_CORE_MODULE
    macro defined, but only the public C API must be tested here. */
+
 #undef Py_BUILD_CORE_MODULE
+/* Always enable assertions */
+#undef NDEBUG
 
 #define PY_SSIZE_T_CLEAN
 
diff --git a/Modules/_testinternalcapi.c b/Modules/_testinternalcapi.c
index 9330e2625b3..1b7563cb20f 100644
--- a/Modules/_testinternalcapi.c
+++ b/Modules/_testinternalcapi.c
@@ -6,6 +6,9 @@
 #  error "Py_BUILD_CORE_BUILTIN or Py_BUILD_CORE_MODULE must be defined"
 #endif
 
+/* Always enable assertions */
+#undef NDEBUG
+
 #define PY_SSIZE_T_CLEAN
 
 #include "Python.h"