How can I use a set the value of a taxonomy term reference field to multiple values?

I have a taxonomy term reference field that allows for up to two values. How can I set both values in a rule?

I set up a workflow in rules that works for setting a single value. I tested this several times and encountered no problems. However, when I changed the "Number of values" for the field from "1" to "2" and then edited the action to add two terms instead of just one, I got the following error message:

    Warning: Missing argument 2 for taxonomy_options_list() in taxonomy_options_list() (line 1375 of /srv/bindings/e87a1e62cfd24fea9dd67cfd074d146c/code/modules/taxonomy/taxonomy.module).
    Warning: Missing argument 3 for taxonomy_options_list() in taxonomy_options_list() (line 1375 of /srv/bindings/e87a1e62cfd24fea9dd67cfd074d146c/code/modules/taxonomy/taxonomy.module).
    Warning: Missing argument 4 for taxonomy_options_list() in taxonomy_options_list() (line 1375 of /srv/bindings/e87a1e62cfd24fea9dd67cfd074d146c/code/modules/taxonomy/taxonomy.module).

When I attempt to set a value by triggering the rule, it fails and the rules debug log has the following output:

    3.235 ms Unable to get a data value. Error: Invalid data value given. Be sure it matches the required data type and format.
    4.254 ms Unable to evaluate action data_set.

What is the correct way to assign two terms at once to a single taxonomy term reference field?