## The symptom
Initializing the gocardless_pro client (2.9.0) with faraday 0.16.2 raised:
```
undefined method `each' for nil:NilClass (NoMethodError)
```
from faraday's `options.rb` during `Faraday.new`. It worked on faraday 0.15.4.
## Why
Not a GoCardless bug: faraday 0.16.x itself had the regression. The reporter
later found it also worked on faraday 0.17.0, and the issue was closed as
superseded.
## The fix
- Upgrade faraday to 0.17.0+ (a maintainer suggested the gem require `~> 0.17`).
- Downgrading to 0.15.4 also works as a stopgap.
## The general lesson
When the stack trace points into a dependency's internals (here faraday's
`options.rb`, not gocardless code) and it worked on a nearby version, bisect
the dependency first. The wrapper library is usually innocent.