Crash on iOS when encoding/decoding nullable nested data classes

[Merkost]

Describe the bug
On real iOS devices, using kotlinx.serialization to encode and decode a @Serializable class containing a nullable nested class crashes the app. This only occurs on physical iOS devices; the same code works on the simulator and on other platforms.

To Reproduce

1. Create a serializable data class with a nullable nested class and encode it using encodeToString.
2. Decode the resulting JSON string with decodeFromString.
3. Run on a physical iOS device.

A minimal reproduction:

@Serializable
data class Outer(
    val id: String,
    val inner: Inner? = null // nullable nested class triggers crash on iOS
)

@Serializable
data class Inner(
    val message: String
)

val json = Json { ignoreUnknownKeys = true }

val encoded = json.encodeToString(Outer.serializer(), Outer("123", Inner("hello")))
// Crash occurs when decodeFromString is called on device:
val decoded = json.decodeFromString(Outer.serializer(), encoded)

The decodeFromString call causes the app to crash on a real iOS device. If inner is made non‑nullable or given a non‑null default instance, the crash disappears.

Expected behavior
encodeToString and decodeFromString should work consistently across platforms. Nullable nested data classes should not cause a crash on iOS.

Environment

• Kotlin version: 2.2.20
• kotlinx.serialization version: 1.9.0
• Kotlin platforms: iOS and Android (Compose Multiplatform)
• IDE: Android Studio 2025.2.1 Canary 5
• iOS: occurs on real devices, not simulator.

Additional context
Providing default non‑null values for the nested classes (e.g., using an empty object) seems to avoid the crash. Removing the nullability from the nested class also avoids the crash.

[pdvrieze]

Does it still trigger if you are not using compose? Have you tried with different compiler versions? It appears this is some compilation problem. Any form of crash logs/information would be valuable too (and was it compiled in debug/release mode)?

[Konstantin-Kttipay]

@pdvrieze It turns out this isn’t a kotlinx.serialization issue. It’s a Kotlin/Native (device-only) crash that’s triggered by string length. On real iOS devices the app crashes once the JSON payload (or any string we pass along) gets beyond ~13x characters (≈135+). The simulator never reproduces it.

So the nullable nested class just made it easier to hit the failure; the actual trigger is string length on K/N arm64. You can reproduce without Compose and even without Serialization by passing/processing a long enough string and the same crash behavior will happen on device.

[sandwwraith]

@Konstantin-Kttipay can you please file a corresponding issue in kotl.in/issue ?

[Konstantin-Kttipay]

Done, here is the ticket on YouTrack

[sandwwraith]

The issue you mentioned is quite old. Is it the correct one?
Anyway, it depends on https://youtrack.jetbrains.com/issue/KT-78089 which will be fixed in 2.3.20.

[Konstantin-Kttipay]

Sorry, here is the correct link
https://youtrack.jetbrains.com/issue/CMP-9088/iOS-crash-when-navigating-with-Serializable-data-class-containing-nullable-argument-with-default-null