diff --git a/source/Octopus.Data/Model/IDocument.cs b/source/Octopus.Data/Model/IDocument.cs
index 997e340..f9d7eec 100644
--- a/source/Octopus.Data/Model/IDocument.cs
+++ b/source/Octopus.Data/Model/IDocument.cs
@@ -2,7 +2,11 @@
 
 namespace Octopus.Data.Model
 {
-    public interface IDocument : IId, INamed
+    public interface IDocument : IDocument<string>, IId
+    {
+    }
+
+    public interface IDocument<out TId> : IId<TId>, INamed
     {
     }
 }
\ No newline at end of file
diff --git a/source/Octopus.Data/Model/IId.cs b/source/Octopus.Data/Model/IId.cs
index 24e0e64..cff0579 100644
--- a/source/Octopus.Data/Model/IId.cs
+++ b/source/Octopus.Data/Model/IId.cs
@@ -2,8 +2,12 @@
 
 namespace Octopus.Data.Model
 {
-    public interface IId
+    public interface IId : IId<string>
     {
-        string Id { get; }
+    }
+
+    public interface IId<out TId>
+    {
+        TId Id { get; }
     }
 }
\ No newline at end of file
diff --git a/source/Octopus.Data/Model/User/IUser.cs b/source/Octopus.Data/Model/User/IUser.cs
index 1c1fb34..7a72ce5 100644
--- a/source/Octopus.Data/Model/User/IUser.cs
+++ b/source/Octopus.Data/Model/User/IUser.cs
@@ -1,9 +1,10 @@
 using System;
 using System.Collections.Generic;
+using Octopus.Server.MessageContracts.Features.Users;
 
 namespace Octopus.Data.Model.User
 {
-    public interface IUser : IId
+    public interface IUser : IId<UserId>
     {
         string Username { get; }
         Guid IdentificationToken { get; }
diff --git a/source/Octopus.Data/Octopus.Data.csproj b/source/Octopus.Data/Octopus.Data.csproj
index fcdb565..277fbb4 100644
--- a/source/Octopus.Data/Octopus.Data.csproj
+++ b/source/Octopus.Data/Octopus.Data.csproj
@@ -15,10 +15,13 @@
     <LangVersion>default</LangVersion>
     <Nullable>enable</Nullable>
     <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
+    <Nullable>enable</Nullable>
+    <LangVersion>8</LangVersion>
   </PropertyGroup>
 
   <ItemGroup>
     <PackageReference Include="Newtonsoft.Json" Version="12.0.3" />
+    <PackageReference Include="Octopus.Server.MessageContracts" Version="0.1.291-enh-asymetry" />
   </ItemGroup>
 
   <ItemGroup>
diff --git a/source/Octopus.Data/Storage/User/IUserStore.cs b/source/Octopus.Data/Storage/User/IUserStore.cs
index e942f48..b8b34f0 100644
--- a/source/Octopus.Data/Storage/User/IUserStore.cs
+++ b/source/Octopus.Data/Storage/User/IUserStore.cs
@@ -1,11 +1,12 @@
 using System;
 using Octopus.Data.Model.User;
+using Octopus.Server.MessageContracts.Features.Users;
 
 namespace Octopus.Data.Storage.User
 {
     public interface IUserStore
     {
-        IUser GetById(string userId);
+        IUser GetById(UserId userId);
         IUser GetByUsername(string username);
         IUser[] GetByEmailAddress(string emailAddress);
         IUser GetByIdentificationToken(Guid identificationToken);