--- a/dom/base/ScriptSettings.cpp
+++ b/dom/base/ScriptSettings.cpp
@@ -371,16 +371,23 @@ AutoJSAPI::InitInternal(nsIGlobalObject*
   if (haveException) {
     JS::Rooted<JS::Value> exn(aCx);
     JS_GetPendingException(aCx, &exn);
 
     JS_ClearPendingException(aCx);
     if (exn.isObject()) {
       JS::Rooted<JSObject*> exnObj(aCx, &exn.toObject());
 
+      // Make sure we can actually read things from it.  This UncheckedUwrap is
+      // safe because we're only getting data for a debug printf.  In
+      // particular, we do not expose this data to anyone, which is very
+      // important; otherwise it could be a cross-origin information leak.
+      exnObj = js::UncheckedUwrap(exnObj);
+      JSAutoCompartment ac(aCx, exnObj);
+
       nsAutoJSString stack, filename, name, message;
       int32_t line;
 
       JS::Rooted<JS::Value> tmp(aCx);
       if (!JS_GetProperty(aCx, exnObj, "filename", &tmp)) {
         JS_ClearPendingException(aCx);
       }
       if (tmp.isUndefined()) {