How to fix "type 'Null' is not a subtype of type ..." error in Flutter

Sunday, June 8, 2025
3 min read

The type ‘Null’ is not a subtype of type … error is common in Flutter when working with models, API responses, or nullable data. This post explains how to fix it and prevent it in your projects.

Why does the “type ‘Null’ is not a subtype of type …” error occur?

This error means you tried to use a value that was null where a non-nullable type was expected. It often happens when:

  • You define a model property as Null or forget to handle null values from APIs.
  • The server returns a value (like a String) but your model expects Null.
  • You access a property or method on a variable that might be null.

Example:

Suppose you have a model like this:

class ServiceTypes {
  Null image;
}

If your API returns a String for image, you’ll get:

type 'String' is not a subtype of type 'Null' in get method flutter

How to fix the “type ‘Null’ is not a subtype of type …” error

Here are the steps to resolve this error:

1. Use the correct type in your models

Update your model to use the expected type:

class ServiceTypes {
  String? image; // Use String? for nullable, or String if always present
}

2. Add null checks before using variables

Before using a variable that might be null, check it:

if (service.image != null) {
  // Safe to use service.image
}

3. Use null-aware operators

Dart provides null-aware operators to make your code safer:

String? image = service.image;
print(image ?? 'No image'); // Use a default value if null

Or, use the null-aware access operator:

service.image?.length;

Best practices to avoid this error

  • Always match your model types to the API response.
  • Use nullable types (String?, int?, etc.) when a value can be null.
  • Validate and check data before using it in your app.
  • Use try-catch blocks when parsing or converting data.

Conclusion

The “type ‘Null’ is not a subtype of type …” error in Flutter is common, but easy to fix with the right approach. By using correct types, adding null checks, and following best practices, you can avoid this error and make your Flutter apps more robust.

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