diff --git a/TeachingRecordSystem/src/TeachingRecordSystem.Api/Endpoints/IdentityWebHooks/GetAnIdentityEndpoints.cs b/TeachingRecordSystem/src/TeachingRecordSystem.Api/Endpoints/IdentityWebHooks/GetAnIdentityEndpoints.cs
index 1f9bde636..bb97b7c97 100644
--- a/TeachingRecordSystem/src/TeachingRecordSystem.Api/Endpoints/IdentityWebHooks/GetAnIdentityEndpoints.cs
+++ b/TeachingRecordSystem/src/TeachingRecordSystem.Api/Endpoints/IdentityWebHooks/GetAnIdentityEndpoints.cs
@@ -105,6 +105,11 @@ IDistributedLockProvider distributedLockProvider
                         Contact.Fields.dfeta_LastIdentityUpdate
                     });
 
+                if (teacher is null)
+                {
+                    throw new InvalidOperationException($"Received a webhook for a teacher that doesn't exist: {user.Trn}.");
+                }
+
                 if (notification.TimeUtc > (teacher.dfeta_LastIdentityUpdate ?? DateTime.MinValue))
                 {
                     await dataverseAdapter.UpdateTeacherIdentityInfo(new UpdateTeacherIdentityInfoCommand()