Guard against error

Error, exact repro not yet clear. Happens in Asserts and when using bluebird promises.

```
TypeError: Cannot read property '0' of undefined
    at /home/ubuntu/public-website/node_modules/clarify/clarify.js:10:35
    at Array.0 (/home/ubuntu/public-website/node_modules/clarify/clarify.js:9:17)
    at TraceModifier._modify (/home/ubuntu/public-website/node_modules/clarify/node_modules/stack-chain/stack-chain.js:41:32)
    at Function.Error.prepareStackTrace (/home/ubuntu/public-website/node_modules/clarify/node_modules/stack-chain/stack-chain.js:81:25)
```