diff --git a/Misc/NEWS b/Misc/NEWS
index 10799600368..c1ab3d381d4 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -15,6 +15,8 @@ Core and Builtins
 - Issue #1688: On Windows, the input() prompt was not correctly displayed if it
   contains non-ascii characters.
 
+- Bug #3951: Py_USING_MEMORY_DEBUGGER should not be enabled by default.
+
 Library
 -------
 
diff --git a/Objects/obmalloc.c b/Objects/obmalloc.c
index fc5ccf7abee..d1586c29f88 100644
--- a/Objects/obmalloc.c
+++ b/Objects/obmalloc.c
@@ -677,8 +677,8 @@ extremely desirable that it be this fast.
 /* This is only useful when running memory debuggers such as
  * Purify or Valgrind.  Uncomment to use.
  *
- */
 #define Py_USING_MEMORY_DEBUGGER
+ */
 
 #ifdef Py_USING_MEMORY_DEBUGGER