Import statement as part of passed in ruleset of a mixin fails

When using mixin like so and passing in an import statement I get some unexpected results.

The import statement inside the passed in declarations throws an error when passed in like so:

.mx({ @import "test2.less"; });

The error message is:

SyntaxError: Cannot read property 'rules' of undefined in ...

However, and I noticed this by chance, adding a colon after the @import will not throw an error, but will not output the imported file contents either:

.mx({ @import: "test2.less"; });

Version: 2.4.0
Testcase: https://gist.github.com/kontur/48fc207097fbfebbfea5
Expected behaviour: @import statement inside the passed in ruleset works. @import statement with consecutive colon fails and throws syntax error.

Use the linked gist and the two mixin examples to reproduce the failure on Less 2.4.0. Trace the ruleset import handling in the parser or evaluator, then verify that an import inside passed declarations produces the imported contents and that the colon variant no longer fails with the reported syntax error.