diff --git a/Misc/NEWS.d/next/Core and Builtins/2021-10-18-22-40-33.bpo-45521.GdMiuW.rst b/Misc/NEWS.d/next/Core and Builtins/2021-10-18-22-40-33.bpo-45521.GdMiuW.rst
new file mode 100644
index 0000000..3a082a4
--- /dev/null
+++ b/Misc/NEWS.d/next/Core and Builtins/2021-10-18-22-40-33.bpo-45521.GdMiuW.rst
@@ -0,0 +1,3 @@
+Fix a bug in the obmalloc radix tree code.  On 64-bit machines, the bug
+causes the tree to hold 46-bits of virtual addresses, rather than the
+intended 48-bits.
diff --git a/Objects/obmalloc.c b/Objects/obmalloc.c
index 15c442b..615703a 100644
--- a/Objects/obmalloc.c
+++ b/Objects/obmalloc.c
@@ -1328,7 +1328,7 @@ _Py_GetAllocatedBlocks(void)
 
 #define MAP_TOP_BITS INTERIOR_BITS
 #define MAP_TOP_LENGTH (1 << MAP_TOP_BITS)
-#define MAP_TOP_MASK (MAP_BOT_LENGTH - 1)
+#define MAP_TOP_MASK (MAP_TOP_LENGTH - 1)
 
 #define MAP_MID_BITS INTERIOR_BITS
 #define MAP_MID_LENGTH (1 << MAP_MID_BITS)