Tests do not run after karma update to v4.0.0

[plentymonkey]

ERROR [karma-server]: TypeError: Cannot read property 'push' of undefined

If I remove Safari from karma.config.jsthe tests run through.

That's how the config looks like:

config.set({
        frameworks: ['jasmine'],
        files: ['test/main.js'],
        preprocessors: {
            'test/main.js': ['webpack', 'sourcemap']
        },
        webpack: require('./config/webpack.test.js'),
        reporters: [
            'progress',
            'kjhtml',
            'coverage-istanbul'
        ],
        autoWatch: true,
        browsers: ['Chrome', 'Firefox', 'Safari'],
...