Should I use cast or List.from?

List.from copies and checks every element once, so errors show up where you parse. cast returns a view that checks on each read, so a bad element can fail later, far from the parsing code.

Does json_serializable avoid this?

Yes. Generated fromJson code converts typed lists for you, which is why Flutter’s JSON guide recommends code generation for medium to large projects.

Can you fix it for me?

If one bug is the whole problem, the App Fix Ticket covers it: a written diagnosis first, then the fix. If the app has more going on, the Launch Check is a written check of the whole app. Each issue found is rated critical, high, medium or low, with where it is and how it was confirmed. No issues worth fixing, no charge. If the Launch Check finds nothing rated medium or higher, Simon refunds the full $450. You keep the report either way. This applies to the first 10 Launch Checks bought after October 30, 2026.

Flutter “List<dynamic> Is Not a Subtype”: Fix

Dart runtime errorFlutter: “type ‘List<dynamic>’ is not a subtype of type ‘List<int>’”

Parsing JSON or reading Firestore data throws type ‘List<dynamic>’ is not a subtype of type ‘List<int>’ (or List<String>, List<Map<String, dynamic>>). jsonDecode and most data APIs build lists as List<dynamic>, even when every item is an int. Assigning that list to a List<int> variable, or using as List<int>, fails at runtime.

Short answer

Convert the list instead of casting it: List<int>.from(json['ids']) or (json['ids'] as List).cast<int>(). For lists of objects, map each element: (json['items'] as List).map((e) => Item.fromJson(e as Map<String, dynamic>)).toList().

Why the list has the wrong type, and the fix

01
A JSON list assigned to a typed list
“The elements iterable itself may have any element type, so this constructor can be used to down-cast a List” Dart, List.from constructor
final List<int> ids = json['ids']; compiles but fails at runtime. List.from creates a new list of the right type and checks every element.
```
final ids = List<int>.from(json['ids'] as List);
final tags = List<String>.from(json['tags'] ?? const []);
```
02
A list of objects
Each element is a Map<String, dynamic>, so convert each one into your model with map and toList.
```
final items = (json['items'] as List)
    .map((e) => Item.fromJson(e as Map<String, dynamic>))
    .toList();
```
03
cast<int>() and mixed numbers
“If any operation tries to read an element that is not an instance of R, the access will throw instead.” Dart, List.cast
cast<int>() returns a typed view without copying. It throws when an element isn’t an int, which happens when the API sends 1.0 or a string. Normalize those first: for 1.0, use (e as num).toInt(); for numeric strings, use int.parse(e as String).
```
final counts = (json['counts'] as List).map((e) => (e as num).toInt()).toList();
```

Checked against the vendor’s docs on November 8, 2026: Dart API: List.from; Dart API: List.cast; Flutter: JSON and serialization.

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Dart runtime errorFlutter: “type ‘List<dynamic>’ is not a subtype of type ‘List<int>’”

Why the list has the wrong type, and the fix

A JSON list assigned to a typed list

A list of objects

cast<int>() and mixed numbers

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