Cognito - CustomSMSSender InvalidCiphertextException: null on Code Decrypt (Golang)

Hi, i followed this document to customize cognito SMS delivery flow https://docs.aws.amazon.com/cognito/latest/developerguide/user-pool-lambda-custom-sms-sender.html

I'm not working on a Javascript environment so wrote this Go snippet:

package main

import (
	"context"
	golog "log"
	"os"

	"github.com/aws/aws-lambda-go/events"
	"github.com/aws/aws-lambda-go/lambda"
	"github.com/aws/aws-sdk-go/aws"
	"github.com/aws/aws-sdk-go/aws/session"
	"github.com/aws/aws-sdk-go/service/kms"
)

// USING THIS TYPES BECAUSE AWS-SDK-GO DOES NOT SUPPORTS THIS

// CognitoEventUserPoolsCustomSmsSender is sent by AWS Cognito User Pools before each mail to send.
type CognitoEventUserPoolsCustomSmsSender struct {
	events.CognitoEventUserPoolsHeader
	Request  CognitoEventUserPoolsCustomSmsSenderRequest  `json:"request"`
}

// CognitoEventUserPoolsCustomSmsSenderRequest contains the request portion of a CustomSmsSender event
type CognitoEventUserPoolsCustomSmsSenderRequest struct {
	UserAttributes map[string]interface{} `json:"userAttributes"`
	Code           string                 `json:"code"`
	ClientMetadata map[string]string      `json:"clientMetadata"`
	Type           string                 `json:"type"`
}

func main() {
	lambda.Start(sendCustomSms)
}

func sendCustomSms(ctx context.Context, event *CognitoEventUserPoolsCustomSmsSender) error {
	golog.Printf("received event=%+v", event)
	golog.Printf("received ctx=%+v", ctx)
	
	config := aws.NewConfig().WithRegion(os.Getenv("AWS_REGION"))
	session, err := session.NewSession(config)
	if err != nil {
		return err
	}

	kmsProvider := kms.New(session)	
	smsCode, err := kmsProvider.Decrypt(&kms.DecryptInput{
		KeyId: aws.String("a8a566c5-796a-4ba1-8715-c9c17c6f0cb5"),
		CiphertextBlob: []byte(event.Request.Code),
	})
	if err != nil {
		return err
	}

	golog.Printf("decrypted code %v", smsCode.Plaintext)
	
	return nil
}

i'm always getting InvalidCiphertextException: : InvalidCiphertextException null, can someone help?

This is how lambda config looks on my user pool:

"LambdaConfig": {
   "CustomSMSSender": {
        "LambdaVersion": "V1_0",
        "LambdaArn": "arn:aws:lambda:eu-west-1:...:function:cognito-custom-auth-sms-sender-dev"
    },
     "KMSKeyID": "arn:aws:kms:eu-west-1:...:key/a8a566c5-796a-4ba1-8715-c9c17c6f0cb5"
},