Fix SF 762891:  "del p[key]" on proxy object raises SystemError()
diff --git a/Objects/weakrefobject.c b/Objects/weakrefobject.c
index e26cb65..f5afb53 100644
--- a/Objects/weakrefobject.c
+++ b/Objects/weakrefobject.c
@@ -389,7 +389,11 @@
 {
     if (!proxy_checkref(proxy))
         return -1;
-    return PyObject_SetItem(PyWeakref_GET_OBJECT(proxy), key, value);
+
+    if (value == NULL)
+        return PyObject_DelItem(PyWeakref_GET_OBJECT(proxy), key);
+    else
+        return PyObject_SetItem(PyWeakref_GET_OBJECT(proxy), key, value);
 }
 
 /* iterator slots */