Flutter json.decode issue <!doctype html> error

I am trying to integrate stripe payment in my Flutter project with backend Laravel. For some reason I am getting error at json.decode.

E/flutter (10999): [ERROR:flutter/lib/ui/ui_dart_state.cc(157)] Unhandled Exception: FormatException: Unexpected character (at character 1) E/flutter (10999): <!doctype html>

FlatButton(
                    shape: RoundedRectangleBorder(
                        borderRadius: BorderRadius.circular(7.0)),
                    color: Colors.redAccent,
                    onPressed: () {
                      var payment = Payment();
                      payment.name = _cardHolderName.text;
                      payment.email = _cardHolderEmail.text;
                      payment.cardNumber = _cardNumber.text;
                      payment.expiryMonth = _expiryMonth.text;
                      payment.expiryYear = _expiryYear.text;
                      payment.cvcNumber = _cvcNumber.text;

                      _makePayment(context, payment);
                      
                    },
                    child: Text('Make Payment',
                        style: TextStyle(color: Colors.white)),
                  ),

void _makePayment(BuildContext context, Payment payment) async {
    PaymentService _paymentService = PaymentService();
    var paymentData = await _paymentService.makePayment(payment);
    var result = json.decode(paymentData.body);

    if (result['result'] == true) {
      CartService _cartService = CartService();
      this.widget.cartItems.forEach((cartItem){
        _cartService.deleteCartItemById(cartItem.productId);
      });
      _showPaymentSuccessMessage(context);
      Timer(Duration(seconds: 2), () {
        Navigator.pop(context);
        Navigator.push(
            context, MaterialPageRoute(builder: (context) => HomeScreen()));
      });
    }
  }

Payment Service

class PaymentService {
  Repository _repository;

  PaymentService(){
    _repository = Repository();
  }

  makePayment(Payment payment) async {
    return await _repository.httpPost('make-payment', payment.toJson());
  }
}

Laravel Code

public function makePayment(Request $request)
    {
        \Stripe\Stripe::setApiKey('sk_test_7422pw1lMsdRfN2FaPmfFKlt00AtiIJD9O');
        
        $token = Token::create([
            'card'=>[
                'number'=>$request-input('cardNumber'),
                'exp_month'=>$request-input('expiryMonth'),
                'exp_year'=>$request-input('expiryYear'),
                'cvc'=>$request-input('cvcNumber')
                ]
            
            ]);

\Stripe\PaymentIntent::create([
  'amount' => 1000,
  'currency' => 'inr',
  'source'  => $token,
  'receipt_email' => $request->input('email'),
]);
    }

Error

E/flutter (10999): [ERROR:flutter/lib/ui/ui_dart_state.cc(157)] Unhandled Exception: FormatException: Unexpected character (at character 1)
E/flutter (10999): <!doctype html>
E/flutter (10999): ^
E/flutter (10999): 
E/flutter (10999): #0      _ChunkedJsonParser.fail (dart:convert-patch/convert_patch.dart:1394:5)
E/flutter (10999): #1      _ChunkedJsonParser.parseNumber (dart:convert-patch/convert_patch.dart:1261:9)
E/flutter (10999): #2      _ChunkedJsonParser.parse (dart:convert-patch/convert_patch.dart:926:22)
E/flutter (10999): #3      _parseJson (dart:convert-patch/convert_patch.dart:31:10)
E/flutter (10999): #4      JsonDecoder.convert (dart:convert/json.dart:495:36)
E/flutter (10999): #5      JsonCodec.decode (dart:convert/json.dart:153:41)
E/flutter (10999): #6      _PaymentScreenState._makePayment (package:factory2homes/screens/payment_screen.dart:160:23)
E/flutter (10999): <asynchronous suspension>
E/flutter (10999): #7      _PaymentScreenState.build.<anonymous closure> (package:factory2homes/screens/payment_screen.dart:142:23)
E/flutter (10999): #8      _InkResponseState._handleTap (package:flutter/src/material/ink_well.dart:706:14)
E/flutter (10999): #9      _InkResponseState.build.<anonymous closure> (package:flutter/src/material/ink_well.dart:789:36)
E/flutter (10999): #10     GestureRecognizer.invokeCallback (package:flutter/src/gestures/recognizer.dart:182:24)
E/flutter (10999): #11     TapGestureRecognizer.handleTapUp (package:flutter/src/gestures/tap.dart:486:11)
E/flutter (10999): #12     BaseTapGestureRecognizer._checkUp (package:flutter/src/gestures/tap.dart:264:5)
E/flutter (10999): #13     BaseTapGestureRecognizer.acceptGesture (package:flutter/src/gestures/tap.dart:236:7)
E/flutter (10999): #14     GestureArenaManager.sweep (package:flutter/src/gestures/arena.dart:156:27)
E/flutter (10999): #15     GestureBinding.handleEvent (package:flutter/src/gestures/binding.dart:222:20)
E/flutter (10999): #16     GestureBinding.dispatchEvent (package:flutter/src/gestures/binding.dart:198:22)
E/flutter (10999): #17     GestureBinding._handlePointerEvent (package:flutter/src/gestures/binding.dart:156:7)
E/flutter (10999): #18     GestureBinding._flushPointerEventQueue (package:flutter/src/gestures/binding.dart:102:7)
E/flutter (10999): #19     GestureBinding._handlePointerDataPacket (package:flutter/src/gestures/binding.dart:86:7)
E/flutter (10999): #20     _rootRunUnary (dart:async/zone.dart:1138:13)
E/flutter (10999): #21     _CustomZone.runUnary (dart:async/zone.dart:1031:19)
E/flutter (10999): #22     _CustomZone.runUnaryGuarded (dart:async/zone.dart:933:7)
E/flutter (10999): #23     _invoke1 (dart:ui/hooks.dart:273:10)
E/flutter (10999): #24     _dispatchPointerDataPacket (dart:ui/hooks.dart:182:5)
E/flutter (10999):