diff --git a/src/proc/mobject/mobject-ref.cpp b/src/proc/mobject/mobject-ref.cpp
index 69b4362f8..e711b26a8 100644
--- a/src/proc/mobject/mobject-ref.cpp
+++ b/src/proc/mobject/mobject-ref.cpp
@@ -46,7 +46,8 @@ namespace mobject {
   /** */
   
   
-  LUMIERA_ERROR_DEFINE (INVALID_PLACEMENTREF, "unresolvable placement reference, or of incompatible type");   
+  LUMIERA_ERROR_DEFINE (INVALID_PLACEMENTREF, "unresolvable placement reference, or of incompatible type");
+  LUMIERA_ERROR_DEFINE (BOTTOM_PLACEMENTREF,  "NIL placement-ID marker encountered.");
   
   
   
diff --git a/src/proc/mobject/placement-ref.hpp b/src/proc/mobject/placement-ref.hpp
index 649fad269..08a42d660 100644
--- a/src/proc/mobject/placement-ref.hpp
+++ b/src/proc/mobject/placement-ref.hpp
@@ -64,6 +64,8 @@ namespace mobject {
   
   
   LUMIERA_ERROR_DECLARE (INVALID_PLACEMENTREF);  ///< unresolvable placement reference, or of incompatible type
+  LUMIERA_ERROR_DECLARE (BOTTOM_PLACEMENTREF);  ///<  NIL placement-ID marker encountered
+  
   
   /**
    * TODO type comment
diff --git a/src/proc/mobject/session/placement-index.hpp b/src/proc/mobject/session/placement-index.hpp
index 928b53ad3..d591d58e9 100644
--- a/src/proc/mobject/session/placement-index.hpp
+++ b/src/proc/mobject/session/placement-index.hpp
@@ -176,6 +176,9 @@ namespace session {
     inline void
     __check_knownID(PlacementIndex const& idx, PlacementMO::ID id)
     {
+    if (!id)
+      throw lumiera::error::Logic ("Encountered a NIL Placement-ID marker"
+                                  ,LUMIERA_ERROR_BOTTOM_PLACEMENTREF);
     if (!idx.contains (id))
       throw lumiera::error::Invalid ("Accessing Placement not registered within the index"
                                     ,LUMIERA_ERROR_NOT_IN_SESSION);              ///////////////////////TICKET #197
diff --git a/tests/components/proc/mobject/placement-ref-test.cpp b/tests/components/proc/mobject/placement-ref-test.cpp
index 55129eb60..14272322e 100644
--- a/tests/components/proc/mobject/placement-ref-test.cpp
+++ b/tests/components/proc/mobject/placement-ref-test.cpp
@@ -22,13 +22,14 @@
 
 
 #include "lib/test/run.hpp"
-#include "lib/lumitime.hpp"
+#include "lib/test/test-helper.hpp"
 #include "proc/mobject/placement.hpp"
 #include "proc/mobject/placement-ref.hpp"
 #include "proc/mobject/session/placement-index.hpp"
 #include "proc/mobject/session/session-service-mock-index.hpp"
 #include "proc/mobject/explicitplacement.hpp"
 #include "proc/mobject/test-dummy-mobject.hpp"
+#include "lib/lumitime.hpp"
 #include "lib/util.hpp"
 
 #include <iostream>
@@ -150,15 +151,9 @@ namespace test    {
 
           // actually, the assignment has invalidated ref1, because of the changed ID
           ASSERT (p1.getID() == p2.getID());
-          try
-            {
-              *ref1;
-              NOTREACHED();
-            }
-          catch (...)
-            {
-              ASSERT (lumiera_error () == LUMIERA_ERROR_INVALID_PLACEMENTREF);
-            }
+          
+          VERIFY_ERROR(INVALID_PLACEMENTREF, *ref1 );
+          
           ASSERT (!index->contains(p1));           // index indeed detected the invalid ref
           ASSERT (3 == ref2.use_count());          // but ref2 is still valid
           
@@ -166,15 +161,14 @@ namespace test    {
           index->remove (ref2);
           ASSERT (!ref2);                          // checks invalidity without throwing
           ASSERT (!refX);
-          try
-            {
-              *ref2;
-              NOTREACHED();
-            }
-          catch (...)
-            {
-              ASSERT (lumiera_error () == LUMIERA_ERROR_INVALID_PLACEMENTREF);
-            }
+          VERIFY_ERROR(INVALID_PLACEMENTREF, *ref2 );
+          
+          // deliberately create an invalid PlacementRef
+          PlacementRef<TestSubMO21> bottom;
+          ASSERT (!bottom);
+          VERIFY_ERROR(INVALID_PLACEMENTREF, *bottom );
+          VERIFY_ERROR(INVALID_PLACEMENTREF, bottom->specialAPI() );
+          VERIFY_ERROR(INVALID_PLACEMENTREF, bottom.resolve() );
 
           //consistency check; then reset PlacementRef index to default
           ASSERT (0 == index->size());
diff --git a/tests/components/proc/mobject/session/placement-index-test.cpp b/tests/components/proc/mobject/session/placement-index-test.cpp
index 63fd5ca32..0e8a42b1b 100644
--- a/tests/components/proc/mobject/session/placement-index-test.cpp
+++ b/tests/components/proc/mobject/session/placement-index-test.cpp
@@ -27,10 +27,10 @@
 //#include "proc/mobject/session.hpp"
 //#include "proc/mobject/session/edl.hpp"
 #include "proc/mobject/session/placement-index.hpp"
-#include "proc/mobject/session/testclip.hpp"
+#include "proc/mobject/session/scope.hpp"
 #include "proc/mobject/placement.hpp"
-//#include "proc/mobject/explicitplacement.hpp"
 #include "lib/util.hpp"
+#include "proc/mobject/session/testclip.hpp"
 
 //#include <boost/format.hpp>
 //#include <iostream>
@@ -137,6 +137,27 @@ namespace test    {
         }
       
       
+      void
+      checkInvalidRef (Idx index)
+        {
+          RefPlacement invalid;
+          PlacementMO::ID invalidID (invalid);
+          ASSERT (!bool(invalidID));
+          ASSERT (!bool(invalid));
+          
+          VERIFY_ERROR(BOTTOM_PLACEMENTREF, index.find(invalid) );
+          VERIFY_ERROR(BOTTOM_PLACEMENTREF, index.find(invalidID) );
+          VERIFY_ERROR(BOTTOM_PLACEMENTREF, index.getScope(invalidID) );
+          
+          ASSERT (!index.contains(invalidID));
+          
+          PMO testObj = TestClip::create();
+          VERIFY_ERROR(INVALID_SCOPE, index.insert(testObj, invalidID) );
+          
+          ASSERT (false == index.remove(invalidID));
+        }
+      
+      
       void
       checkScopeHandling (Idx index)
         {