TypeError: Cannot read property 'customer_id' of undefined

Deployed as per the example. But keep on getting these errors from calling the functions from index.html:

createStripeCharge
```
TypeError: Cannot read property 'customer_id' of undefined
at exports.createStripeCharge.functions.firestore.document.onCreate (/srv/index.js:35:34)
at
at process._tickDomainCallback (internal/process/next_tick.js:228:7)
```

addPaymentSource
```
TypeError: Cannot read property 'customer_id' of undefined
at exports.addPaymentSource.functions.firestore.document.onCreate (/srv/index.js:73:38)
at
at process._tickDomainCallback (internal/process/next_tick.js:228:7)
```
To exclude any rules issues, I've allowed read and write firestore.rules to anyone with uid.

Any idea what could be causing these errors?