Calculating percentage of trials completed

Hi,

Every 50 trials participants are shown a feedback item telling them they can take a self-timed break. In the same item, I would like to show them what percentage of trials they have completed. I tried to do this the following way: a JavaScript item before the sequence that contains this in the Prepare tab:

vars.total_trials = 958;
vars.count_trials = (vars.count_trial_sequence) + 1;

Then, before the feedback item I added another JavaScript item with this in the Run tab:

vars.completed = (vars.count_trials / vars.total_trials) * 100;

In the feedback item, I put the text "You have completed [completed] % of trials.

However, when I tested the experiment in the browser, I always got "You have completed NaN % of trials". Is this because the two variables I defined are integers and the "completed" variable is a float? Or do I need to place the JavaScript items in different locations? I could change the feedback item to saying that they have completed X trials out of 958 by using using count_trial_sequence but I'd prefer to use percentages.

Thanks!