diff --git a/machine.capnp b/machine.capnp
index e40e201..173c263 100644
--- a/machine.capnp
+++ b/machine.capnp
@@ -7,6 +7,7 @@ using CSharp = import "programming_language/csharp.capnp";
 $CSharp.namespace("FabAccessAPI.Schema");
 
 using General = import "general.capnp";
+using Optional = General.Optional;
 using User = import "user.capnp".User;
 using Space = import "space.capnp".Space;
 
@@ -21,9 +22,9 @@ struct Machine {
         totakeover @6;
     }
     struct MachineInfoExtended {
-        currentUser @0 :User;
-        lastUser @1 :User;
-        instructorUser @2 :User;
+        currentUser @0 :Optional(User);
+        lastUser @1 :Optional(User);
+        instructorUser @2 :Optional(User);
     }
 
     struct Reservation {
@@ -37,7 +38,7 @@ struct Machine {
     name @2 :Text;
     description @3 :Text;
     state @4 :MachineState;
-    manager @5:User;
+    manager @5:Optional(User);
     wiki @13 :Text;
     urn @14 :Text;
     category @15 :Text;