jquery split() and indexOf results in "Object doesn't support this property or method"

I have the following code:

var selected = $('#hiddenField').val().split(",");
...
if (selected.indexOf(id) > 0) {
   ... set value ...
}

I'm dynamically creating a CheckBoxList, and trying to remember the state of the checkboxes by putting the selected IDs into the hidden field.

I get an error stating that "Object doesn't support this property or method". My assumption is that selected is an array, which should support indexOf. Is that incorrect?