diff --git a/src/Authentication/Exceptions/RegisterUserProvisioningException.cs b/src/Authentication/Exceptions/RegisterUserProvisioningException.cs
new file mode 100644
index 000000000..9dc55daf6
--- /dev/null
+++ b/src/Authentication/Exceptions/RegisterUserProvisioningException.cs
@@ -0,0 +1,33 @@
+using System;
+
+namespace Altinn.Platform.Authentication.Exceptions
+{
+    /// <summary>
+    /// Thrown when self-identified user provisioning against register fails, so that the
+    /// OIDC sign-in flow aborts with a clear error instead of continuing with an unpopulated user.
+    /// </summary>
+    public class RegisterUserProvisioningException : Exception
+    {
+        /// <summary>
+        /// Empty constructor.
+        /// </summary>
+        public RegisterUserProvisioningException() : base()
+        {
+        }
+
+        /// <summary>
+        /// With message.
+        /// </summary>
+        public RegisterUserProvisioningException(string message) : base(message)
+        {
+        }
+
+        /// <summary>
+        /// With message and inner exception.
+        /// </summary>
+        public RegisterUserProvisioningException(string message, Exception innerException)
+            : base(message, innerException)
+        {
+        }
+    }
+}
diff --git a/src/Authentication/Services/OidcServerService.cs b/src/Authentication/Services/OidcServerService.cs
index 3cf22733b..d0fb0363f 100644
--- a/src/Authentication/Services/OidcServerService.cs
+++ b/src/Authentication/Services/OidcServerService.cs
@@ -22,6 +22,7 @@
 using Altinn.Platform.Authentication.Core.RepositoryInterfaces;
 using Altinn.Platform.Authentication.Core.Services.Interfaces;
 using Altinn.Platform.Authentication.Enum;
+using Altinn.Platform.Authentication.Exceptions;
 using Altinn.Platform.Authentication.Helpers;
 using Altinn.Platform.Authentication.Model;
 using Altinn.Platform.Authentication.Services.Interfaces;
@@ -312,7 +313,22 @@ public async Task<UpstreamCallbackResult> HandleUpstreamCallback(UpstreamCallbac
             // ===== 2) Exchange upstream code for upstream tokens =====
             OidcProvider provider = ChooseProviderByKey(upstreamTx.Provider);
             UserAuthenticationModel userIdenity = await ExtractUserIdentityFromUpstream(input, upstreamTx, provider, cancellationToken);
-            userIdenity = await IdentifyOrCreateAltinnUser(userIdenity, provider);
+
+            try
+            {
+                userIdenity = await IdentifyOrCreateAltinnUser(userIdenity, provider);
+            }
+            catch (RegisterUserProvisioningException ex)
+            {
+                _logger.LogError(ex, "Self-identified user provisioning via register failed; aborting sign-in.");
+                return new UpstreamCallbackResult
+                {
+                    Kind = UpstreamCallbackResultKind.LocalError,
+                    StatusCode = StatusCodes.Status502BadGateway,
+                    LocalErrorMessage = "User provisioning failed. Please try again later."
+                };
+            }
+
             AddLocalScopes(userIdenity);
 
             // 3. Create or refresh Altinn session session
@@ -1520,11 +1536,6 @@ private async Task<UserAuthenticationModel> IdentifyOrCreateAltinnUser(UserAuthe
                         email: null,
                         CancellationToken.None);
 
-                    if (provisioned is null)
-                    {
-                        return userAuthenticationModel;
-                    }
-
                     userAuthenticationModel.UserID = (int)provisioned.User.Value.UserId.Value;
                     userAuthenticationModel.PartyID = (int)provisioned.PartyId.Value;
                     userAuthenticationModel.PartyUuid = provisioned.Uuid;
@@ -1576,11 +1587,6 @@ private async Task<UserAuthenticationModel> IdentifyOrCreateAltinnUser(UserAuthe
                         userAuthenticationModel.Email,
                         CancellationToken.None);
 
-                    if (provisioned is null)
-                    {
-                        return userAuthenticationModel;
-                    }
-
                     userAuthenticationModel.UserID = (int)provisioned.User.Value.UserId.Value;
                     userAuthenticationModel.PartyID = (int)provisioned.PartyId.Value;
                     userAuthenticationModel.PartyUuid = provisioned.Uuid;
@@ -1628,7 +1634,7 @@ private async Task<UserAuthenticationModel> IdentifyOrCreateAltinnUser(UserAuthe
             return userAuthenticationModel;
         }
 
-        private async Task<SelfIdentifiedUser?> GetOrCreateSelfIdentifiedUserViaRegister(
+        private async Task<SelfIdentifiedUser> GetOrCreateSelfIdentifiedUserViaRegister(
             SelfIdentifiedUserType selfIdentifiedUserType,
             string externalIdentity,
             string userName,
@@ -1647,9 +1653,8 @@ private async Task<UserAuthenticationModel> IdentifyOrCreateAltinnUser(UserAuthe
 
             if (response is null)
             {
-                _logger.LogError(
-                    "Register self-identified provisioning returned no result for externalIdentity {ExternalIdentity}; sign-in cannot complete.",
-                    externalIdentity);
+                throw new RegisterUserProvisioningException(
+                    $"Register self-identified provisioning returned no result for externalIdentity '{externalIdentity}'; sign-in cannot complete.");
             }
 
             return response;