problem extracting an app.asar file using the asar command

Question

I am new to Electron and want to make (at first) a sample javascript change in the Slack Electron app. I am here on a Mac OSX:

/Applications/Slack.app/Contents/Resources

and run the following:

asar extract app.asar ~/Desktop/src

And then without even making a change run:

asar pack ~/Desktop/src app.asar

The "re-packed" file is significantly larger:

    -rw-r--r--   1 oliverwilliams  staff  28336486 Apr 21 03:25 app.asar
    -rw-r--r--   1 oliverwilliams  staff   8175963 Mar 21 13:26 app.asar.original

and Slack will not start. Reverting back to the original file, Slack runs again.

Obviously I need to crawl before I can walk. What needs to be done here?