$ gulp build
[12:51:23] Using gulpfile ~/Work/stash/stash_engine/gulpfile.js
[12:51:23] Starting 'build'...
[12:51:23] Starting 'clean'...
[12:51:23] Finished 'clean' after 13 ms
[12:51:23] Starting 'scss-lint'...
[12:51:23] Starting 'js-lint'...
[12:51:23] Starting 'sass'...
[12:51:23] Starting 'useref'...
[12:51:23] Starting 'icons'...
[12:51:23] Starting 'copy-images'...
[12:51:23] Starting 'copy-fonts'...
events.js:167
      throw er; // Unhandled 'error' event
      ^

Error: Error: File not found with singular glob: /Users/dmoles/Work/stash/stash_engine/ui-library/css/main.css (if this was purposeful, use `allowEmpty` option)
    at DestroyableTransform.<anonymous> (/Users/dmoles/Work/stash/stash_engine/node_modules/gulp-useref/index.js:67:32)
    at DestroyableTransform.emit (events.js:182:13)
    at Pumpify.emit (events.js:182:13)
    at Pumpify.Duplexify._destroy (/Users/dmoles/Work/stash/stash_engine/node_modules/duplexify/index.js:191:15)
    at /Users/dmoles/Work/stash/stash_engine/node_modules/duplexify/index.js:182:10
    at process._tickCallback (internal/process/next_tick.js:61:11)
Emitted 'error' event at:
    at DestroyableTransform.onerror (/Users/dmoles/Work/stash/stash_engine/node_modules/vinyl-fs/node_modules/readable-stream/lib/_stream_readable.js:558:12)
    at DestroyableTransform.emit (events.js:182:13)
    at DestroyableTransform.<anonymous> (/Users/dmoles/Work/stash/stash_engine/node_modules/gulp-useref/index.js:67:18)
    at DestroyableTransform.emit (events.js:182:13)
    [... lines matching original stack trace ...]
    at process._tickCallback (internal/process/next_tick.js:61:11)