Cannot read property 'substring' of undefined

[aritudo1]

Keeps saying undefined here event.detail.source.substring. Not sure what needs to be defined. I've tried substituting with different strings and also defining the source. Using this on GovCloud and Cloudwatch rule to collect CloudTrail snapshot events and invoke the Step Function. Any help is appreciated.

var volumeId = event.detail.source.substring(event.detail.source.indexOf('/') + 1);
var snapshotId = event.detail.snapshot_id.substring(event.detail.snapshot_id.indexOf('/') + 1);

{
"errorType": "TypeError",
"errorMessage": "Cannot read property 'substring' of undefined",
"trace": [
"TypeError: Cannot read property 'substring' of undefined",
" at Runtime.exports.handler (/var/task/TagSnapshots.js: 12:40)",
" at Runtime.handleOnce (/var/runtime/Runtime.js:66:25)"
]
}