String could not be parsed as XML

When I run:

```
php ./vendor/bin/mgf --show-source app/code/core/
```

I observe the following exception:

```
[Exception] 
String could not be parsed as XML

mgf [--patterns-dir [PATTERNS-DIR]] [--show-source] [--] <path>
```

Environment: Mac, PHP 5.6.16, Magento 1.9.2.2. 
