Is it safe to put DKIM key in _middleware.ts?

I’m trying to follow the instructions as per here: MailChannels · Cloudflare Pages docs

It shows that you need to add some info to the personalizations object, in particular this one:

      "dkim_private_key": context.env.DKIM_PRIVATE_KEY

Whenever I do that, the deployment fails with this:

17:10:49.186	✨ Compiled Worker successfully
17:10:49.217	Validating asset output directory
17:10:49.850	Deploying your site to Cloudflare's global network...
17:10:54.054	 Uploading... (631/631)
17:10:54.057	✨ Success! Uploaded 0 files (631 already uploaded) (0.73 sec)
17:10:54.645	✨ Upload complete!
17:10:56.144	Success: Assets published!
17:10:56.950	Error: Failed to publish your Function. Got error: Uncaught ReferenceError: context is not defined
  at functionsWorker-0.047060805545406925.js:892:27

I can solve the problem by putting the key directly into _middleware.ts and not relying on the environment variable (which I have done verbatim to the instructions). I’d rather fix it properly but without much free time, is the key in _middleware.ts ok for now?