diff --git a/Include/objimpl.h b/Include/objimpl.h
index 9a27ec384f9..3f21b70e074 100644
--- a/Include/objimpl.h
+++ b/Include/objimpl.h
@@ -265,7 +265,7 @@ extern PyGC_Head *_PyGC_generation0;
 #define _PyGCHead_REFS(g) ((g)->gc.gc_refs >> _PyGC_REFS_SHIFT)
 #define _PyGCHead_SET_REFS(g, v) do { \
     (g)->gc.gc_refs = ((g)->gc.gc_refs & ~_PyGC_REFS_MASK) \
-        | (v << _PyGC_REFS_SHIFT); \
+        | (((size_t)(v)) << _PyGC_REFS_SHIFT);             \
     } while (0)
 #define _PyGCHead_DECREF(g) ((g)->gc.gc_refs -= 1 << _PyGC_REFS_SHIFT)
 
diff --git a/Misc/NEWS b/Misc/NEWS
index 91c669382ce..dd5d7da83b8 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -8,6 +8,8 @@ What's New in Python 3.4.1?
 Core and Builtins
 -----------------
 
+- Issue #20929: Add a type cast to avoid shifting a negative number.
+
 - Issue #20731: Properly position in source code files even if they
   are opened in text mode. Patch by Serhiy Storchaka.