type 'Null' is not a subtype of type 'String' in type cast

[ielb]

Getting this error when using a post method
this is TrackTrace : #0 CastMap.forEach. (dart:_internal/cast.dart:288:25)
#1 _LinkedHashMapMixin.forEach (dart:collection-patch/compact_hash.dart:400:8)
#2 CastMap.forEach (dart:_internal/cast.dart:287:13)
#3 mapToQuery (package:http/src/utils.dart:17:7)
#4 Request.bodyFields= (package:http/src/request.dart:137:12)
#5 BaseClient._sendUnstreamed (package:http/src/base_client.dart:87:17)
#6 BaseClient.post (package:http/src/base_client.dart:32:7)
#7 post. (package:http/http.dart:69:16)
#8 _withClient (package:http/http.dart:164:20)
#9 post (package:http/http.dart:68:5)
#10 Api.httpPost (package:knorrieplay/services/api.dart:33:14)
#11 UserService.addUser (package:knorrieplay/services/user_service.dart:8:18)
#12 AuthProvider.signInWithFacebook (package:knorrieplay/providers/auth_provider.dart:219:23)

#13 _LoginPageState.facebookAuth (package:knorrieplay/pages/login_page.dart:89:18)

the method i used
Future<http.Response?> httpPost(String endPath, Object body) async {
try {
Uri uri = Uri.parse('$url$endPath');
return http.post(uri, body: body, headers: headers);
} catch (e) {
print(e);
}
}

::::::::::: when i use dio it works fine

[natebosch]

The doc comment on http.post says:

body sets the body of the request. It can be a String, a List<int> or a Map<String, String>.

You have passed a Map which contains a value that is null, which is not a String.