diff --git a/Octokit.Reactive/Clients/ObservableAssigneesClient.cs b/Octokit.Reactive/Clients/ObservableAssigneesClient.cs
index beabc06d..de7eb9c9 100644
--- a/Octokit.Reactive/Clients/ObservableAssigneesClient.cs
+++ b/Octokit.Reactive/Clients/ObservableAssigneesClient.cs
@@ -25,6 +25,9 @@ namespace Octokit.Reactive
         /// <returns></returns>
         public IObservable<User> GetAllForRepository(string owner, string name)
         {
+            Ensure.ArgumentNotNullOrEmptyString(owner, "owner");
+            Ensure.ArgumentNotNullOrEmptyString(name, "name");
+
             return GetAllForRepository(owner, name, ApiOptions.None);
         }