[CRASH] Cannot read property 'length' of undefined

[zdnk]

This is my dangerfile.js

import { danger, message, warn, fail } from 'danger'
var exec = require('child_process').execSync
var _ = require('lodash')

var result;

try {
    result = exec("swiftlint lint --quiet --reporter json").stdout.toString()
} catch(ex) {
    result = ex.stdout.toString()
}

var json = JSON.parse(result)
json.forEach(finding => {
    if (finding.severity == undefined || finding.reason == undefined) {
        return
    }

    var severity = _.toLower(finding.severity)
    if (severity == "warning" && finding.reason) {
        warn(finding.reason)
    } else if (severity == "error" && finding.reason) {
        fail(finding.reason)
    }
})

The error:

Error:  TypeError: Cannot read property 'length' of undefined
    at Executor.<anonymous> (.../node_modules/danger/distribution/runner/Executor.js:191:31)
    at step (.../node_modules/danger/distribution/runner/Executor.js:32:23)
    at Object.next (.../node_modules/danger/distribution/runner/Executor.js:13:53)
    at .../node_modules/danger/distribution/runner/Executor.js:7:71
    at new Promise (<anonymous>)
    at __awaiter (.../node_modules/danger/distribution/runner/Executor.js:3:12)
    at Executor.handleResultsPostingToSTDOUT (.../node_modules/danger/distribution/runner/Executor.js:175:16)
    at Executor.<anonymous> (.../node_modules/danger/distribution/runner/Executor.js:160:26)
    at step (.../node_modules/danger/distribution/runner/Executor.js:32:23)
    at Object.next (.../node_modules/danger/distribution/runner/Executor.js:13:53)
error An unexpected error occurred: "Command failed.

[zdnk]

It happens when there is a lot of warns/fails (like in the loop I have in my file).
It happens even if I just use warn("dd") and fail("dd") in the loop.