Combobox in PDF populated with Javascript keeps resetting to zero

Question

I'm trying to put a combobox (named "Additional Users") on a PDF form that populates with values from an array. According to this help page, it looks like you can use a 2D array to store the keys and values for the combobox.

I've placed the below code on the combobox as a custom calculation script. It will populate the combobox, but once you select an option and click away from it, the value resets to the first option, which in this case is 0. What am I doing wrong?

var dropdown = this.getField("Additional Users");
dropdown.setItems([ ["0", 0], ["5", 5], ["10", 10], ["15", 15] ]);