A String is an object

IPython/html/static/widgets/js/widget.js

@@ -236,7 +236,7 b' define(["widgets/js/manager",'
                     packed.push(that._pack_models(sub_value));
                 });
                 return packed;
-            } else if (value instanceof Date) {
+            } else if (value instanceof Date || value instanceof String) {
                 return value;
             } else if (value instanceof Object) {
                 packed = {};