diff --git a/src/proc/mobject/placement-ref.hpp b/src/proc/mobject/placement-ref.hpp
index 37f629472..5677aeac4 100644
--- a/src/proc/mobject/placement-ref.hpp
+++ b/src/proc/mobject/placement-ref.hpp
@@ -106,6 +106,13 @@ namespace mobject {
           validate(id_); 
         }
       
+      /** Default is an NIL Placement ref. It throws on any access. */
+      PlacementRef ()
+        : id_()
+        {
+          REQUIRE (!isValid(), "hash-ID clash with existing ID");
+        }
+      
       PlacementRef (PlacementRef const& r)    ///< copy ctor
         : id_(r.id_)
         { 
diff --git a/src/proc/mobject/session/scope-path.hpp b/src/proc/mobject/session/scope-path.hpp
index f5366a457..5835e6a8d 100644
--- a/src/proc/mobject/session/scope-path.hpp
+++ b/src/proc/mobject/session/scope-path.hpp
@@ -43,7 +43,7 @@ namespace session {
    */
   class ScopePath
     {
-      vector<RefPlacement> path_;
+      vector<Scope> path_;
       
     public:
       ScopePath();
diff --git a/src/proc/mobject/session/scope.cpp b/src/proc/mobject/session/scope.cpp
index 6c00a222e..abd0293ff 100644
--- a/src/proc/mobject/session/scope.cpp
+++ b/src/proc/mobject/session/scope.cpp
@@ -42,6 +42,13 @@ namespace session {
   }
   
   
+  Scope::Scope ()
+    : anchor_()
+  {
+    REQUIRE (!anchor_.isValid());
+  }
+  
+  
   ScopeLocator::ScopeLocator()
     : focusStack_(new QueryFocusStack)
   {
diff --git a/src/proc/mobject/session/scope.hpp b/src/proc/mobject/session/scope.hpp
index ce3de7d86..bc192bbb5 100644
--- a/src/proc/mobject/session/scope.hpp
+++ b/src/proc/mobject/session/scope.hpp
@@ -59,6 +59,7 @@ namespace session {
       
     public:
       Scope (PlacementMO const& constitutingPlacement);
+      Scope (); ///< unlocated NIL scope
       
       static Scope const& containing (PlacementMO const& aPlacement);              //////////////TODO really returning a const& here??
       static Scope const& containing (RefPlacement const& refPlacement);