send image with base64

[mftlhn]

thanks for the great library. I have one issue that I can't send the image. I has try this https://github.com/kiwi-bop/flutter_signature_pad/blob/master/example/lib/main.dart#L98-L103 but in my server, I can't decode it. The way I decode it like this $image = base64_decode($data->assign_sa); but then I have error
InvalidArgumentException: Malformed UTF-8 characters, possibly incorrectly encoded in file /home/serc1112/public_html/api-sat/vendor/laravel/framework/src/Illuminate/Http/JsonResponse.php on line 91
am I wrong how to decode it?