actix/actix · error · panic

Got unknown value

Error message

Got unknown value: {:?}

What it means

SystemRegistry::get panics when the registry holds a boxed value for this actor's TypeId that cannot be downcast to Addr<A>. Since the registry only inserts Addr<A> for type A, hitting this means internal invariant corruption — the stored entry's concrete type does not match the requested key type. It indicates a bug or unsafe manipulation of the registry.

Solutions

1. Audit any code that inserts into the registry to ensure it only inserts Box::new(addr.clone()) of Addr<A> under TypeId::of::<A>().
2. Check for duplicate versions of the actix crate in your dependency tree (cargo tree -d) causing TypeId confusion.
3. Avoid unsafe insertion into SystemRegistry; use the public get/set API only.
4. If it persists, file a bug with a minimal reproduction since this should be unreachable.

// Check before relying on registry contents
let known = reg.registry.contains_key(&TypeId::of::<A>());

// Panic is not catchable in safe patterns; ensure only public API inserts into registry
let addr = SystemRegistry::get::<MyService>(); // starts or returns existing

When it happens

Trigger: Calling SystemRegistry::get::<A>() when reg.registry contains an entry keyed by TypeId::of::<A>() whose downcast_ref::<Addr<A>>() returns None — only possible if a non-Addr<A> Box<dyn Any> was inserted under A's TypeId.

Common situations: Custom registry manipulation or unsafe code inserting foreign boxes; version mismatches where the same crate appears twice (distinct TypeIds for what looks like the same actor); memory corruption from unsafe code.

Understand the failure class

Background: "This is a bug, please report it": internal invariant violations, unreachable panics, and SNH errors explained — this error's family across 47 libraries.

Related errors

• Actor already started
• Message type should always be M
• wrong return type for message
• Mocker actor used before set
• Map must not be polled after it returned `Poll::Ready`

AI-assisted analysis of actix/actix@36e5d97e41 (2026-09-11). Data as JSON: /api/errors/76c9c8799708c54e. Report an issue: GitHub.

Appendix: source

Thrown at actix/src/registry.rs:303

        addr
    }
}

impl SystemRegistry {
    pub(crate) fn new(system: ArbiterHandle) -> Self {
        Self {
            system,
            registry: HashMap::default(),
        }
    }

    /// Return address of the service. If service actor is not running
    /// it get started in the system.
    pub fn get<A: SystemService + Actor<Context = Context<A>>>(&mut self) -> Addr<A> {
        if let Some(addr) = self.registry.get(&TypeId::of::<A>()) {
            match addr.downcast_ref::<Addr<A>>() {
                Some(addr) => return addr.clone(),
                None => panic!("Got unknown value: {:?}", addr),
            }
        }

        let addr = A::start_service(&self.system);
        self.registry
            .insert(TypeId::of::<A>(), Box::new(addr.clone()));
        addr
    }

    /// Check if actor is in registry, if so, return its address
    pub fn query<A: SystemService + Actor<Context = Context<A>>>(&self) -> Option<Addr<A>> {
        if let Some(addr) = self.registry.get(&TypeId::of::<A>()) {
            match addr.downcast_ref::<Addr<A>>() {
                Some(addr) => return Some(addr.clone()),
                None => return None,
            }
        }

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