diff --git a/src/lib/p.hpp b/src/lib/p.hpp
index a3ca96975..8bb46aacb 100644
--- a/src/lib/p.hpp
+++ b/src/lib/p.hpp
@@ -50,6 +50,7 @@
 #define LUMIERA_P_H
 
 
+#include "lib/error.hpp"
 #include <tr1/memory>
 
 
@@ -107,19 +108,19 @@ namespace lumiera {
   
       template<typename _O_>
       friend inline bool
-      operator<  (P const& p, P<_O_> const& q) { return (p && q) && (*p < *q); }   ////TICKET #307  : problem with equality test in associative containers, where equal(a,b) := !(a < b) && !(b < a)
+      operator<  (P const& p, P<_O_> const& q) { REQUIRE (p && q); return *p < *q; }   ///< @note deliberately not allowing comparison on NIL ////TICKET #307  : problem with equality test in associative containers, where equal(a,b) := !(a < b) && !(b < a)
   
       template<typename _O_>
       friend inline bool
-      operator>  (P const& p, P<_O_> const& q) { return (p && q) && (*q < *p); }
+      operator>  (P const& p, P<_O_> const& q) { REQUIRE (p && q); return *q < *p; }
 
       template<typename _O_>
       friend inline bool
-      operator<= (P const& p, P<_O_> const& q) { return (p && q)? (*p <= *q) : (!p && !q); }
+      operator<= (P const& p, P<_O_> const& q) { REQUIRE (p && q); return *p <= *q;}
 
       template<typename _O_>
       friend inline bool
-      operator>= (P const& p, P<_O_> const& q) { return (p && q)? (*p >= *q) : (!p && !q); }
+      operator>= (P const& p, P<_O_> const& q) { REQUIRE (p && q); return *p >= *q;}
       
     };
    
diff --git a/tests/lib/customsharedptrtest.cpp b/tests/lib/customsharedptrtest.cpp
index 1c15f25e5..5c8817dce 100644
--- a/tests/lib/customsharedptrtest.cpp
+++ b/tests/lib/customsharedptrtest.cpp
@@ -22,6 +22,7 @@
 
 
 #include "lib/test/run.hpp"
+#include "lib/test/test-helper.hpp"
 #include "lib/util.hpp"
 
 #include "lib/p.hpp"
@@ -37,6 +38,8 @@ namespace asset {
     using lumiera::P;
     using std::tr1::shared_ptr;
     using std::tr1::weak_ptr;
+            
+    using lumiera::error::LUMIERA_ERROR_ASSERTION;
     
     
     struct X 
@@ -288,17 +291,28 @@ namespace asset {
             
             ASSERT (!(pXX == pX5));    // compare subtype ptr to empty ptr: "unequal but not orderable"
             ASSERT ( (pXX != pX5));
-            ASSERT (!(pXX <  pX5));
-            ASSERT (!(pXX >  pX5));
-            ASSERT (!(pXX <= pX5));
-            ASSERT (!(pXX >= pX5));
             
             ASSERT ( (pX5 == pX6));    // compare two empty ptrs: "equal, equivalent but not orderable"
             ASSERT (!(pX5 != pX6));
-            ASSERT (!(pX5 <  pX6));
-            ASSERT (!(pX5 >  pX6));
-            ASSERT ( (pX5 <= pX6));
-            ASSERT ( (pX5 >= pX6));
+            
+            // order relations on NIL pointers disallowed
+            
+#if false ///////////////////////////////////////////////////////////////////////////////////////////////TICKET #537 : restore throwing ASSERT
+            VERIFY_ERROR (ASSERTION, pXX <  pX5 );
+            VERIFY_ERROR (ASSERTION, pXX >  pX5 );
+            VERIFY_ERROR (ASSERTION, pXX <= pX5 );
+            VERIFY_ERROR (ASSERTION, pXX >= pX5 );
+            
+            VERIFY_ERROR (ASSERTION, pX5 <  pXX );
+            VERIFY_ERROR (ASSERTION, pX5 >  pXX );
+            VERIFY_ERROR (ASSERTION, pX5 <= pXX );
+            VERIFY_ERROR (ASSERTION, pX5 >= pXX );
+            
+            VERIFY_ERROR (ASSERTION, pX5 <  pX6 );
+            VERIFY_ERROR (ASSERTION, pX5 >  pX6 );
+            VERIFY_ERROR (ASSERTION, pX5 <= pX6 );
+            VERIFY_ERROR (ASSERTION, pX5 >= pX6 );
+#endif    ///////////////////////////////////////////////////////////////////////////////////////////////TICKET #537 : restore throwing ASSERT
           }
       };