Elixir New Added 12 September 2026

Elixir: protocol Enumerable not implemented for nil of type Atom

Something handed `Enum` a nil where a list was expected, and nil has no Enumerable implementation. The real fault is upstream: an Ecto association that was never preloaded, a `Map.get` that missed, or a function whose error branch returns nil instead of an empty list.

Quick fix

Read the commands before running them. Anything that restarts a service, deletes data or changes permissions should be tried on a non-production system first.

Quick fix

# Ecto is the usual source: an unloaded association is a struct, not a list
iex> post.comments
#Ecto.Association.NotLoaded<association :comments is not loaded>

Repo.get!(Post, id) |> Repo.preload(:comments)
from(p in Post, preload: [:comments]) |> Repo.all()

# Make the empty case explicit rather than letting nil travel
Enum.map(list || [], &process/1)

# Or match it, so the failure lands where the value was produced
case fetch_items() do
  {:ok, items} -> Enum.map(items, &process/1)
  {:error, reason} -> Logger.error("no items: #{inspect(reason)}")
end

# The same message for a struct means the struct needs a real implementation,
# usually by deriving it or by calling Map.to_list/1 first

How to diagnose Elixir errors

Elixir errors are shaped by the actor model: a GenServer.call timeout does not mean the server crashed, it means the server was busy for longer than the caller was willing to wait. The right question is usually "what is that process doing?" rather than "why did the call fail?". Because supervisors restart failed processes automatically, transient errors can also hide in the logs while the system appears healthy.

If the quick fix above does not resolve it, work through these steps. They apply to this whole class of error, not just to this one message, which is usually what saves the time.

Attach to a running node with iex --remsh and inspect the process: Process.info(pid, :message_queue_len). A growing mailbox means the process is the bottleneck.
Use :observer.start() to see the supervision tree, process memory and message queues visually.
Move long-running work out of handle_call. Use handle_cast, a Task, or a dedicated pool so the GenServer stays responsive.
Check restart intensity. A supervisor that exceeds max_restarts takes down its own supervisor, producing a cascade that looks like an unrelated failure at the top.
For compile errors in dependencies, run mix deps.compile --force and check that any required native toolchain (make, gcc, erlang headers) is installed.

Tools worth reaching for

:observer.start()
iex --remsh
Process.info/2
mix deps.tree
:recon

Authoritative references

Primary documentation for this error, worth reading before applying any fix in production.

hexdocs.pm hexdocs.pm

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