Where exactly (which line) does the crash occur ? var marketMap = [String: SomeMarket].init(minimumCapacity: markets.count) and marketMap[market.symbol] = market there are two cases occurred according to callstack from firebase,

The code you have shown cannot be a proof that your issue is not a multithreading problem. Especially, using Promise makes me feel that your issue is a multithreading problem. Anyway, if you have any clues about what might be causing your issue other than multithreading, it is your decision on which possibility you put high priority. Thanks for your advice we'll inspect code more carefully

typically, multithreading issue always crash with EXC_BAC_ACCESS, but EXC_BREAKPOINT in this context Another issue with no multithreading issue definitely libswiftCore.dylib swift_slowAlloc.cold.1 + 16 1 libswiftCore.dylib swift_slowAlloc + 208 2 libswiftCore.dylib swift_allocObject + 52 3 libswiftCore.dylib static _DictionaryStorage.allocate(scale:age:seed:) + 152 4 libswiftCore.dylib Dictionary.init(minimumCapacity:) + 72     func fetchContent() -> Promise<SomeContent> {         Service.getMarketsV2().map { markets in             var marketMap = [String: SomeMarket].init(minimumCapacity: markets.count)             for market in markets where !market.parentMarket.isEmpty {                 marketMap[market.symbol] = market             }             return SomeContent(marketMap: marketMap, markets: markets)         }     }

here is the code for event in events { &#9;&#9;try outputContent?.marketMap[event.symbol]?.update(with: event) }