Flutter and Dart errorFlutter: “type 'Null' is not a subtype of type 'String'”

Where the wrong value comes from, and the fix

1. 01

   A JSON field that is missing or null

   “If you have a typo when accessing a nonexistent JSON field, your code throws an error during runtime.” Flutter, JSON and serialization

   jsonDecode() returns dynamic, so the compiler can’t see that json['name'] might be null. Assigning it to a String field, or casting it with as String, fails at runtime. A typo in the key gives the same error, because a missing key reads as null.

   // throws when name is missing or null
   name: json['name'] as String,
   
   // handles it
   name: json['name'] as String? ?? '',

2. 02

   A generated model that is stricter than the data

   With json_serializable or freezed, a field declared as String makes the generated fromJson cast it with as String. Regenerate after changing the model, and declare fields the API can leave out as String?, or give them a default with @JsonKey(defaultValue: '').

3. 03

   A Firestore document without the field

   “The index [] operator on the Map class returns null if the key isn't present.” Dart, Understanding null safety

   doc.data()!['title'] is null for any document written before the field existed, or written by a code path that skips it. Old documents are the usual reason it fails for some users and not others. Read the field as nullable with a fallback, or add the field to the old documents.

   final data = doc.data()!;
   final title = data['title'] as String? ?? 'Untitled';

4. 04

   The value is there, but it is a different type

   “If you aren't sure that the object is of type T, then use is T to check the type before using the object.” Dart, Operators: type test operators

   type 'int' is not a subtype of type 'String' means the API sent 42 where the model expects '42'. '_Map<String, dynamic>' is not a subtype of type 'List<dynamic>' means it sent one object where the code expects a list. Check the type with is before using the value, or change the model to match the real response.

   final idText = json['id'].toString(); // 42 or '42' both work
   final tags = json['tags'] is List ? json['tags'] as List : const [];

5. 05

   Route arguments that were not passed

   “Use the as operator to cast an object to a particular type if and only if you are sure that the object is of that type.” Dart, Operators: type test operators

   ModalRoute.of(context)!.settings.arguments as String is a cast, and it throws on null when a screen opens without arguments, for example from a deep link or a pushNamed call that leaves them out. Read them as String? and handle null, or pass the value through the screen’s constructor instead.

Checked against the vendor’s docs on November 8, 2026: Flutter: JSON and serialization; Dart: Operators (type test operators); Dart: Understanding null safety (Map lookups); Dart API: TypeError class.

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