Unable to fix CSSLint error "Expected RBRACE" on CSS grid rule

Question

I'm using Stylus and Grunt with CSS Lint and am getting an error on this rule:

.example
  grid-template-columns repeat(auto-fill, minmax(50%, 1fr))

The error I get is:

>> ERROR: Expected RBRACE at line 9, col 147897. This rule looks for recoverable syntax errors. (errors) Browsers: All

Adding line exceptions as in the following (as described in the docs) doesn't get rid of the error:

.example
  /* csslint ignore:start */
  grid-template-columns repeat(auto-fill, minmax(50%, 1fr))
  /* csslint ignore:end */

Not sure how to ignore the rule inline or in the .csslintrc file either since I can't determine what rule is being broken. Any ideas?