diff -r 000000000000 -r 6474c204b198 js/src/jit-test/tests/auto-regress/bug720396.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/js/src/jit-test/tests/auto-regress/bug720396.js	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,11 @@
+// |jit-test| error:TypeError
+
+// Binary: cache/js-dbg-64-1962ca549264-linux
+// Flags:
+//
+
+var g = newGlobal();
+var dbg1 = new Debugger;
+var gw1 = dbg1.addDebuggee(g);
+var dbg2 = new Debugger;
+var gw2 = dbg2.addDebuggee((gw1.__proto__));