[Solved] Unexpected character (at character) Error in Flutter

In this post, we are going to show you how to solve "Unexpected character (at character)" error in Flutter/Dart. This error usually happens while decoding an invalid JSON string, or if you are getting invalid JSON from REST API in the Dio package.

Error Message:

══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY ╞═════════════════
The following FormatException was thrown building Builder:
Unexpected character (at character 25)

This error happens when you are decoding invalid JSON with json.decode() or jsonDecode().

I/flutter (16776): DioError [DioErrorType.other]: 
FormatException: Unexpected character (at character 1)
I/flutter (16776): <br />
I/flutter (16776): ^
I/flutter (16776):
I/flutter (16776): Source stack:
I/flutter (16776): #0      DioMixin.fetch (package:dio/src/dio_mixin.dart:473:35)
I/flutter (16776): #1      DioMixin.request (package:dio/src/dio_mixin.dart:468:12)
I/flutter (16776): #2      DioMixin.get (package:dio/src/dio_mixin.dart:55:12)

This error happens when you are getting invalid JSON data from REST API. You need to remind, that Dio() package returns decoded objects from JSON strings. 

Cause of Error:

The cause of this error is an invalid JSON string, either JSON string inside code lines or JSON string data fetched from REST API.

How to Solve the Error:

Case 1: If you are using jsonDecode() or json.decode() to decode the JSON string, you need to verify if the JSON string is invalid. For example:

String jsonstring = '{"error":false,"msg":"",aa"data":[{"name":"Canada","capital":"Ottawa"}]}';
var jsonobj = jsonDecode(jsonstring); // Runtime Error: Unexpected character (at character)

To solve this error let's verify if the JSON string is valid or not. You can use any JSON validator. I'm using https://jsonlint.com/ and I got the error like below when validating the JSON.

Error: Parse error on line 3:
...false,	"msg": "",	aa "data": [{		"nam
---------------------^
Expecting 'STRING', got 'undefined'

Its means, I have an invalid JSON string, for example, there is "aa", an unexpected character after "msg" in the JSON string above code.

You can escape this error with try...catch:

try{
 var jsonobj = jsonDecode(jsonstring);
}catch(e){
  print(e);
}

Case 2: If you are fetching JSON string from REST API, or the internet, you may get an error message generated by the compiler in server-side while generating the JSON string. You can use API tester software to check if the REST API is returning valid JSON, For example:

Here, after checking the response from REST API with API tester software, I got the message like below:

This means, we got the error from the server-side, and the error message is returned by REST API along with JSON string. You can resolve this error on a server-side script or, disable the error message on the compiler.

You can escape this error from Dio() like below:

try{
  response = await dio.get(url);
}catch(e){
  print(e);
}

In this way, you can resolve the "Unexpected character (at character)" Error in Flutter/Dart.