diff --git a/src/Identity/Controllers/AccountsController.cs b/src/Identity/Controllers/AccountsController.cs
index aba344aeda..f80274d65d 100644
--- a/src/Identity/Controllers/AccountsController.cs
+++ b/src/Identity/Controllers/AccountsController.cs
@@ -1,7 +1,4 @@
-// FIXME: Update this file to be null safe and then delete the line below
-#nullable disable
-
-using System.Text;
+using System.Text;
 using Bit.Core;
 using Bit.Core.Auth.Enums;
 using Bit.Core.Auth.Models.Api.Request.Accounts;
@@ -41,7 +38,7 @@ public class AccountsController : Controller
     private readonly IFeatureService _featureService;
     private readonly IDataProtectorTokenFactory<RegistrationEmailVerificationTokenable> _registrationEmailVerificationTokenDataFactory;
 
-    private readonly byte[] _defaultKdfHmacKey = null;
+    private readonly byte[]? _defaultKdfHmacKey = null;
     private static readonly List<UserKdfInformation> _defaultKdfResults =
     [
         // The first result (index 0) should always return the "normal" default.
@@ -147,7 +144,7 @@ public class AccountsController : Controller
         User user = model.ToUser();
 
         // Users will either have an emailed token or an email verification token - not both.
-        IdentityResult identityResult = null;
+        IdentityResult? identityResult = null;
 
         // PM-28143 - Just use the MasterPasswordAuthenticationData.MasterPasswordAuthenticationHash
         string masterPasswordHash = model.MasterPasswordAuthentication?.MasterPasswordAuthenticationHash
@@ -159,14 +156,14 @@ public class AccountsController : Controller
                 identityResult = await _registerUserCommand.RegisterUserViaEmailVerificationToken(
                     user,
                     masterPasswordHash,
-                    model.EmailVerificationToken);
+                    model.EmailVerificationToken!);
                 return ProcessRegistrationResult(identityResult, user);
 
             case RegisterFinishTokenType.OrganizationInvite:
                 identityResult = await _registerUserCommand.RegisterUserViaOrganizationInviteToken(
                     user,
                     masterPasswordHash,
-                    model.OrgInviteToken,
+                    model.OrgInviteToken!,
                     model.OrganizationUserId);
                 return ProcessRegistrationResult(identityResult, user);
 
@@ -174,23 +171,23 @@ public class AccountsController : Controller
                 identityResult = await _registerUserCommand.RegisterUserViaOrganizationSponsoredFreeFamilyPlanInviteToken(
                     user,
                     masterPasswordHash,
-                    model.OrgSponsoredFreeFamilyPlanToken);
+                    model.OrgSponsoredFreeFamilyPlanToken!);
                 return ProcessRegistrationResult(identityResult, user);
 
             case RegisterFinishTokenType.EmergencyAccessInvite:
                 identityResult = await _registerUserCommand.RegisterUserViaAcceptEmergencyAccessInviteToken(
                     user,
                     masterPasswordHash,
-                    model.AcceptEmergencyAccessInviteToken,
-                    (Guid)model.AcceptEmergencyAccessId);
+                    model.AcceptEmergencyAccessInviteToken!,
+                    (Guid)model.AcceptEmergencyAccessId!);
                 return ProcessRegistrationResult(identityResult, user);
 
             case RegisterFinishTokenType.ProviderInvite:
                 identityResult = await _registerUserCommand.RegisterUserViaProviderInviteToken(
                     user,
                     masterPasswordHash,
-                    model.ProviderInviteToken,
-                    (Guid)model.ProviderUserId);
+                    model.ProviderInviteToken!,
+                    (Guid)model.ProviderUserId!);
                 return ProcessRegistrationResult(identityResult, user);
 
             default:
diff --git a/test/Identity.Test/Controllers/AccountsControllerTests.cs b/test/Identity.Test/Controllers/AccountsControllerTests.cs
index 86d407256e..90ef917cf6 100644
--- a/test/Identity.Test/Controllers/AccountsControllerTests.cs
+++ b/test/Identity.Test/Controllers/AccountsControllerTests.cs
@@ -11,7 +11,6 @@ using Bit.Core.Entities;
 using Bit.Core.Enums;
 using Bit.Core.Exceptions;
 using Bit.Core.KeyManagement.Models.Api.Request;
-using Bit.Core.KeyManagement.Models.Data;
 using Bit.Core.Models.Data;
 using Bit.Core.Repositories;
 using Bit.Core.Services;
@@ -963,7 +962,7 @@ public class AccountsControllerTests : IDisposable
 
         // Act & Assert
         var ex = await Assert.ThrowsAsync<BadRequestException>(() => _sut.PostRegisterFinish(model));
-        Assert.Equal("KdfType couldn't be found on either the MasterPasswordUnlockData or the Kdf property passed in.", ex.Message);
+        Assert.Equal("KdfType couldn't be found on either the MasterPasswordUnlock or the Kdf property passed in.", ex.Message);
     }
 
     [Theory, BitAutoData]
@@ -1001,7 +1000,7 @@ public class AccountsControllerTests : IDisposable
 
         // Act & Assert
         var ex = await Assert.ThrowsAsync<BadRequestException>(() => _sut.PostRegisterFinish(model));
-        Assert.Equal("KdfIterations couldn't be found on either the MasterPasswordUnlockData or the KdfIterations property passed in.", ex.Message);
+        Assert.Equal("KdfIterations couldn't be found on either the MasterPasswordUnlock or the KdfIterations property passed in.", ex.Message);
     }
 
     [Theory, BitAutoData]
@@ -1192,7 +1191,7 @@ public class AccountsControllerTests : IDisposable
 
         // Act & Assert
         var ex = Assert.Throws<BadRequestException>(() => model.Validate(ctx).ToList());
-        Assert.Equal("Master password hash and hash are not equal.", ex.Message);
+        Assert.Equal("AuthenticationData MasterPasswordHash and root level MasterPasswordHash provided and are not equal. Only provide one.", ex.Message);
     }
 
     private void SetDefaultKdfHmacKey(byte[]? newKey)