http://dvcs.w3.org/hg/IndexedDB/raw-file/tip/Overview.html#widl-IDBObjectStore-add says the second (key) parameter is optional when calling add(), but if specified it must not be null. Mozilla correctly throws a DATA_ERR (test: http://mxr.mozilla.org/mozilla2.0/source/dom/indexedDB/test/test_key_requirements.html?force=1#196 ) but WebKit does not throw.
Created attachment 89698 [details] test case
This is fixed by the patch to bug 58613. *** This bug has been marked as a duplicate of bug 58613 ***