Getting list of taxonomy slugs using modifier

Hi, I'm trying to get a comma separated list of my taxonomy terms for an entry, so that I can then pass that as a filter to a collection.

My entry data has the taxonomy stored like so:

product_categories:
 - category_a
 - category_b

currently I am trying to do

{{product_categories|list}}

using the list modifier which should return a comma separated list? but get :

 ErrorException
 implode(): Invalid arguments passed

Where am I going wrong, or have I misunderstood that modifiers can be used to format taxonomy lists?