Upgrading from 1.4.1 to 1.5.0 causes crash on cache bust

[PLPeeters]

When updating from version 1.4.1 to 1.5.0, the task suddenly stopped working with the following warning message:

Warning: Cannot use 'in' operator to search for 'index.html' in undefined Use --force to continue.

My configuration is as follows:

{
    index: {
        options: {
            assets: ['scripts/**', 'views/**', '../libs/**', 'styles/**'],
            queryString: true,
            baseDir: 'angular'
        },
        src: ['angular/index.html']
    }
}

Relevant directory structure:

angular/
    scripts/
    views/
    ...
GruntFile.js
...

Reverting to 1.4.1 fixes the issue.