diff --git a/API/Data/Repositories/MangaFileRepository.cs b/API/Data/Repositories/MangaFileRepository.cs
index 32a8bfc34..89c6bb418 100644
--- a/API/Data/Repositories/MangaFileRepository.cs
+++ b/API/Data/Repositories/MangaFileRepository.cs
@@ -5,6 +5,7 @@ using API.Entities;
 using Microsoft.EntityFrameworkCore;
 
 namespace API.Data.Repositories;
+#nullable enable
 
 public interface IMangaFileRepository
 {
diff --git a/API/Helpers/Builders/KoreaderBookDtoBuilder.cs b/API/Helpers/Builders/KoreaderBookDtoBuilder.cs
index 5a5834adb..debbe0347 100644
--- a/API/Helpers/Builders/KoreaderBookDtoBuilder.cs
+++ b/API/Helpers/Builders/KoreaderBookDtoBuilder.cs
@@ -1,3 +1,4 @@
+using System;
 using System.Security.Cryptography;
 using System.Text;
 using API.DTOs.Koreader;
@@ -39,7 +40,7 @@ public class KoreaderBookDtoBuilder : IEntityBuilder<KoreaderBookDto>
     public KoreaderBookDtoBuilder WithDeviceId(string installId, int userId)
     {
         var hash = SHA256.HashData(Encoding.UTF8.GetBytes(installId + userId));
-        _dto.Device_id = hash.ToString();
+        _dto.Device_id = Convert.ToHexString(hash);
         return this;
     }
 }