Invalid argument(s): The given string does not have the correct begin marker expected in a PEM file

When I try to run the example from the source code, whenever it reaches the very first line in the main class:
var credential = Credentials.applicationDefault();

it triggers the Unhandled exception shown below.
I'm using the "service-account.json" file approach.
Could you please guide me on what to look for on this issue?

Thanks

Invalid argument(s): The given string does not have the correct begin marker expected in a PEM file.
#0 parsePem (package:x509/src/x509_base.dart:257:7)
#1 _SyncIterator.moveNext (dart:core-patch/core_patch.dart:183:26)
#2 Iterable.first (dart:core/iterable.dart:489:13)
#3 new Certificate.fromJson (package:firebase_admin/src/auth/credential.dart:42:34)
#4 new ServiceAccountCredential.fromJson (package:firebase_admin/src/auth/credential.dart:82:35)
#5 new ServiceAccountCredential (package:firebase_admin/src/auth/credential.dart:88:41)
#6 Credentials._credentialFromFile (package:firebase_admin/src/credential.dart:190:14)
#7 Credentials._getApplicationDefault (package:firebase_admin/src/credential.dart:138:14)
#8 Credentials.applicationDefault (package:firebase_admin/src/credential.dart:77:33)
#9 main (file:///D:/OneDrive%20-%20Softtek/AppDev/firebase_admin/example/main.dart:11:32)
#10 _delayEntrypointInvocation. (dart:isolate-patch/isolate_patch.dart:283:19)
#11 _RawReceivePortImpl._handleMessage (dart:isolate-patch/isolate_patch.dart:184:12)