Empty karma html report - jasmine

Question

Here are the facts:
I have a nodejs app
I've written some Jasmine tests
I'm using karma to generate the code coverage report
Here is the karma.conf.js:
module.exports = function (config) {
config.set({
frameworks: ['jasmine', 'browserify'],
files: [
'utils/dates.js',
'utils/maths.js',
'spec/server_utils_spec.js'
],
preprocessors: {
'utils/*.js': ['coverage', 'browserify'],
'spec/server_utils_spec.js': ['coverage', 'browserify']
},
reporters: ['progress', 'coverage'],
coverageReporter: {
type: 'html',
dir: 'coverage/'
},
colors: true,
logLevel: config.LOG_INFO,
autoWatch: true,
singleRun: false
})
}
These are my packages (I know they are global):
→ npm -g list | grep -i "karma"
├─┬ karma#2.0.4
├─┬ karma-browserify#5.3.0
├─┬ karma-coverage#1.1.2
├─┬ karma-html-reporter#0.2.7
├── karma-jasmine#1.1.2
├─┬ karma-mocha#1.3.0
and despite the terminal log
/usr/bin/karma start server/karma.conf.js
23 07 2018 15:39:36.033:INFO [framework.browserify]: registering rebuild (autoWatch=true)
23 07 2018 15:39:36.817:INFO [framework.browserify]: 3073 bytes written (0.03 seconds)
23 07 2018 15:39:36.817:INFO [framework.browserify]: bundle built
23 07 2018 15:39:36.818:WARN [karma]: No captured browser, open http://localhost:9876/
23 07 2018 15:39:36.822:INFO [karma]: Karma v2.0.4 server started at http://0.0.0.0:9876/
23 07 2018 15:39:39.135:INFO [Chrome 67.0.3396 (Linux 0.0.0)]: Connected on socket H9J96y-fFn-7PfkHAAAA with id manual-9692
Chrome 67.0.3396 (Linux 0.0.0): Executed 0 of 2 SUCCESS (0 secs / 0 secs)
Chrome 67.0.3396 (Linux 0.0.0): Executed 1 of 2 SUCCESS (0 secs / 0.001 secs)
Chrome 67.0.3396 (Linux 0.0.0): Executed 2 of 2 SUCCESS (0 secs / 0.002 secs)
Chrome 67.0.3396 (Linux 0.0.0): Executed 2 of 2 SUCCESS (0.033 secs / 0.002 secs)
TOTAL: 2 SUCCESS
I get this result when I open the generated html report:
I've tried the solutions to all the other related questions and I've come up with nothing.
My questions:
why the empty report?
how can I manage to have the report
Thank you in advance!