# Troubleshooting
## `Tried to fetch a resource, but the resource does not exist.`
This is the most common issue you will face as a new user of Specs.
This panic will occur whenever a `System` is first dispatched, and one or
more of the components and/or resources it uses is missing from `World`.
There are a few main reasons for this occurring:
* Forgetting to call `setup` after building a `Dispatcher` or `ParSeq`. Make
sure this is always run before the first dispatch.
* Not adding mandatory resources to `World`. You can usually find these by
searching for occurrences of `ReadExpect` and `WriteExpect`.
* Manually requesting components/resources from `World` (not inside a `System`),
where the component/resource is not used by any `System`s, which is most common
when using the `EntityBuilder`. This is an artifact of how `setup` works, it
will only add what is found inside the used `System`s.
If you use other components/resources, you need to manually register/add these
to `World`.