nautechsystems/nautilus_trader · error

Initialize event missing topics: expected 4, was {topics_len

Error message

Initialize event missing topics: expected 4, was {topics_len}

What it means

parse_initialize_event_hypersync requires 4 topics for a Uniswap V4 Initialize event (event signature, poolId, currency0, currency1 — the latter two indexed). Fewer topics means the log is not a V4 Initialize event, so the parser cannot proceed.

Source

Thrown at crates/adapters/blockchain/src/exchanges/parsing/uniswap_v4/initialize.rs:92

    let block_number = extract_block_number(&log)?;

    // The pool address for V4 is the PoolManager contract address (the event emitter)
    let pool_manager_address = Address::from_slice(
        log.address
            .clone()
            .expect("PoolManager address should be set in logs")
            .as_ref(),
    );

    // Extract currency0 and currency1 from topics
    // topics[0] = event signature
    // topics[1] = poolId (bytes32)
    // topics[2] = currency0 (indexed)
    // topics[3] = currency1 (indexed)
    let topics = &log.topics;
    if topics.len() < 4 {
        anyhow::bail!(
            "Initialize event missing topics: expected 4, was {}",
            topics.len()
        );
    }

    // Extract Pool ID from topics[1] - this is the unique identifier for V4 pools
    let pool_id_bytes = topics[1]
        .as_ref()
        .ok_or_else(|| anyhow::anyhow!("Missing poolId topic"))?
        .as_ref();
    let pool_identifier = Ustr::from(&hex::encode_prefixed(pool_id_bytes));

    let currency0 = Address::from_slice(
        topics[2]
            .as_ref()
            .ok_or_else(|| anyhow::anyhow!("Missing currency0 topic"))?
            .as_ref()
            .get(12..32)

View on GitHub (pinned to 18893faf8b)

Solutions

1. Filter hypersync queries by the exact V4 Initialize topic0 and require 4 topics
2. Check topics.len() >= 4 in the caller before invoking the parser and skip otherwise
3. Verify the emitting contract is a V4 PoolManager
4. If V3 Initialize logs are mixed in, route them to the V3 parser instead

Example fix

// before
if topics.len() < 4 {
    anyhow::bail!("Initialize event missing topics: expected 4, was {}", topics.len());
}
// after
if topics.len() < 4 {
    tracing::debug!(n = topics.len(), "skipping non-V4 initialize log");
    return Ok(None); // or continue
}

fn is_v4_initialize_topics(log: &HypersyncLog) -> bool {
    log.topics.len() >= 4 && log.topics[0].as_ref() == INITIALIZE_V4_TOPIC0
}
if !is_v4_initialize_topics(&log) { skip_or_log(); }

match parse_initialize_event_hypersync(&log, &dex) {
    Ok(ev) => handle(ev),
    Err(e) => { tracing::debug!(%e, "initialize log skipped"); Ok(None) }
}

When it happens

Trigger: Calling parse_initialize_event_hypersync with a log carrying fewer than 4 topics — non-V4 PoolManager logs, a broad hypersync filter without topic0, or malformed synthetic logs.

Common situations: Subscribing without an event-signature topic filter so unrelated events hit the parser; decoding logs from V3 pools (Initialize has 0 indexed currencies) or other AMMs; test logs missing indexed params.

Understand the failure class

Background: "missing required argument" and "the following required arguments were not provided": what required-argument errors mean and how to fix them — this error's family across 20 libraries.

Related errors

• Initialize event missing topics: expected 4, was {log_topics
• Missing fee in topic3 when parsing pool created event
• Initialize event data too short: expected at least 160 bytes
• Missing data in the pool created event log
• HyperSync parsing of burn event is not defined in this dex:

AI-assisted analysis of nautechsystems/nautilus_trader@18893faf8b (2026-09-08). Data as JSON: /api/errors/c7dc846159560583. Report an issue: GitHub.