Adaptive Cards render on skype and emulator, but not web chat

I have a added a few Adaptive Cards to my bot. They render perfectly in the emulator and, when deployed, render perfectly in Skype and Slack. However, in the web chat, I get the following response instead of the rendered card:

[File of type 'application/vnd.microsoft.card.adaptive']

Any thoughts on what the problem might be? I'm uncertain how to troubleshoot when the functionality works in all the other environments.

I'm use the Node version of the bot framework.

via HardlyNeutral