Apex Rest Callouts

I have written this code : public class AnimalLocator {

public static String getAnimalNameById(Integer i) {

        Http http = new Http();

        HttpRequest request = new HttpRequest();

        request.setEndpoint('https://th-apex-http-callout.herokuapp.com/animals'+i);

        request.setMethod('GET');

        HttpResponse response = http.send(request); 

    //if the request is successful, parse the JSON response

    if(response.getStatusCode()==200){

        Map<String,Object> result = (Map<String,Object>)JSON.deserializeUntyped(response.getBody());

    Map<String,Object> animal = (Map<String, Object>) result.get('animal');

    return string.valueOf(animal.get('name'));

    }else{

        return null;

    }

    }

}

and I am getting this error

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