engine: "slimerjs" Aborted due to warnings - 0.4.1

I've encountered the same type of problem reported here: https://github.com/iamchrismiller/grunt-casper/issues/55. I've also tried uninstalling & reinstalling grunt-casper fresh, but I'm still running into the same issue.

Running tests with phantomjs works fine, but with slimerjs I get:

Running "casper:fast" (casper) task
Casper Task 'casper:fast' took ~245ms to run
Warning: Task "casper:fast" failed. Use --force to continue.
Aborted due to warnings.

with no other description.

Any ideas or things to try? I would really like to be able to run my tests in both engines, thanks!

Edit - Here is the full output with --verbose:

Spawning casperjs with args:  [ 'test',
  '00-helpers.js',
  '--engine=slimerjs',
  '--showconsolelogs=false',
  '--fail-fast=true',
  '--ignore-ssl-errors=true',
  '--pre=tests/pre-test.js',
  '--user=casper_fast',
  '--pass=casper' ] 
Found PhantomJS Executable /Users/justin/Documents/Pathbrite/node_modules/grunt-casper/node_modules/phantomjs/lib/phantom/bin/phantomjs 
Found SlimerJS Executable /Users/justin/Documents/Pathbrite/node_modules/grunt-casper/node_modules/slimerjs/lib/slimer/slimerjs 
Found CasperJS Executable /Users/justin/Documents/Pathbrite/node_modules/grunt-casper/node_modules/casperjs/bin/casperjs>> 
Casper Task 'casper:fast' took ~150ms to run
Warning: Task "casper:fast" failed. Use --force to continue.
Aborted due to warnings.

iamchrismiller / grunt-casper

engine: "slimerjs" Aborted due to warnings - 0.4.1 #61