diff --git a/ext/bigdecimal/bigdecimal.c b/ext/bigdecimal/bigdecimal.c
index ecd43fdc5c..c442335a0a 100644
--- a/ext/bigdecimal/bigdecimal.c
+++ b/ext/bigdecimal/bigdecimal.c
@@ -14,6 +14,7 @@
 #include "ruby/util.h"
 
 #ifndef BIGDECIMAL_DEBUG
+# undef NDEBUG
 # define NDEBUG
 #endif
 #include <assert.h>
diff --git a/ext/date/date_core.c b/ext/date/date_core.c
index ccac90a32e..e44dcc15e2 100644
--- a/ext/date/date_core.c
+++ b/ext/date/date_core.c
@@ -11,6 +11,7 @@
 #include <sys/time.h>
 #endif
 
+#undef NDEBUG
 #define NDEBUG
 #include <assert.h>