What causes "Error: Unable to calculate `midpoint` (no bid or ask)" in nautechsystems/nautilus_trader?

Guard with a check that both bid and ask sides are non-empty before calling the midpoint FFI function.

nautechsystems/nautilus_trader · critical

Error: Unable to calculate `midpoint` (no bid or ask)

Error message

Error: Unable to calculate `midpoint` (no bid or ask)

What it means

This panic fires in the FFI wrapper `orderbook_midpoint` when `OrderBook::midpoint()` returns None because the book does not contain at least one bid and one ask, so the midpoint is undefined. The `abort_on_panic` wrapper turns the panic into a process abort rather than a recoverable FFI error.

Source

Thrown at crates/model/src/ffi/orderbook/book.rs:319

/// # Panics
///
/// Panics if unable to calculate spread (requires at least one bid and one ask).
#[unsafe(no_mangle)]
pub extern "C" fn orderbook_spread(book: &mut OrderBook) -> f64 {
    abort_on_panic(|| {
        book.spread()
            .expect("Error: Unable to calculate `spread` (no bid or ask)")
    })
}

/// # Panics
///
/// Panics if unable to calculate midpoint (requires at least one bid and one ask).
#[unsafe(no_mangle)]
pub extern "C" fn orderbook_midpoint(book: &mut OrderBook) -> f64 {
    abort_on_panic(|| {
        book.midpoint()
            .expect("Error: Unable to calculate `midpoint` (no bid or ask)")
    })
}

/// # Panics
///
/// Panics if `order_side` is `NoOrderSide`.
#[unsafe(no_mangle)]
#[cfg_attr(feature = "high-precision", allow(improper_ctypes_definitions))]
pub extern "C" fn orderbook_get_avg_px_for_quantity(
    book: &mut OrderBook,
    qty: Quantity,
    order_side: OrderSideOptional,
) -> f64 {
    book.get_avg_px_for_quantity(
        qty,
        order_side
            .as_option()
            .expect("Order side must be Buy or Sell"),

View on GitHub (pinned to 18893faf8b)

Solutions

Guard with a check that both bid and ask sides are non-empty before calling the midpoint FFI function.
Defer midpoint calculations until a full two-sided snapshot has been applied.
Compute the midpoint from optional best bid/ask in caller code, returning None when either side is absent.
Ensure subscriptions deliver both bid and ask quotes/depth for the instrument.

Example fix

// before (Python FFI caller)
mid = orderbook_midpoint(book)
// after
if has_bid_orders(book) and has_ask_orders(book):
    mid = orderbook_midpoint(book)
else:
    mid = None

Defensive patterns

Strategy: validation

Validate before calling

def can_compute_midpoint(book) -> bool:
    return book.bids_len() > 0 and book.asks_len() > 0

Type guard

def is_two_sided(book) -> bool:
    return len(book.bids()) > 0 and len(book.asks()) > 0

Prevention

Defer midpoint math until both sides are populated
Treat empty books as an expected state and query optional accessors instead of FFI panicking ones
Remember FFI panics abort the process — they cannot be caught as exceptions

When it happens

Trigger: Calling `orderbook_midpoint(book)` when the bid or ask side is empty — partial quotes, pre-snapshot books, or books cleared by a reset.

Common situations: Warm-up periods before market data lands; one-sided liquidity; data feeds that publish only trades; querying after a `clear` in response to a gap fill.

Understand the failure class

Background: EmptyResultError / "no results found": when an API or scraper succeeds but returns zero rows — this error's family across 9 libraries.

Related errors

AI-assisted analysis of nautechsystems/nautilus_trader@18893faf8b (2026-09-08). Data as JSON: /api/errors/7512740e309c129b. Report an issue: GitHub.