How to keep Hangouts from adding a "1" before SMS shortcode?

I frequently use SMS shortcodes to get my account balance from my bank. I send a text message to the bank's short code (in this case, 728464), and my bank replies with a text message containing my current account balance. I've been doing this for a while now with no issues.

I recently switched phones (to a Nexus 6 running Android 5.0.1), and now when I send my text to 728464, I get an error (from my phone, not from the bank) saying that the short code I am texting is not valid. I noticed in the error that either Hangouts or Android seem to be prepending a "1" to the short code (see the screenshot). I'm guessing that this has something to do with it.

How can I prevent Hangouts from adding this "1" to my short code (which seems to be preventing me from sending the SMS)?

In case it's pertinent, I have imported all of my SMS conversations from my old phone using SMS Backup Restore app, and have selected Hangouts as my default SMS application.