Index: generic/nsfInt.h
===================================================================
diff -u -re48161ab167236344f7d23da3e855c31d1a1ebc7 -r958b24f921951c5e7c3f454134ddf17843df22bc
--- generic/nsfInt.h	(.../nsfInt.h)	(revision e48161ab167236344f7d23da3e855c31d1a1ebc7)
+++ generic/nsfInt.h	(.../nsfInt.h)	(revision 958b24f921951c5e7c3f454134ddf17843df22bc)
@@ -353,9 +353,17 @@
 # include <alloca.h>
 #endif
 
+#ifdef PRE9
+# define NSF_PLAUSIBLE_REFCOUNT(A) ((A)->refCount >= 0)
+# define NSF_PLAUSIBLE_LENGTH(A) ((A)->length >= 0)
+#else
+# define NSF_PLAUSIBLE_REFCOUNT(A) ((A)->refCount < (size_t)-100)
+# define NSF_PLAUSIBLE_LENGTH(A) ((A)->length < (size_t)-100)
+#endif
+
 #if !defined(NDEBUG)
 # define ISOBJ(o) ((o) != NULL && ISOBJ_(o))
-# define ISOBJ_(o) ((o) != (void*)0xdeadbeaf && (((o)->typePtr != NULL) ? ((o)->typePtr->name != NULL) : ((o)->bytes != NULL)) && (((o)->bytes != NULL) ? (o)->length >= 0 : 1) && (o)->refCount >= 0u)
+# define ISOBJ_(o) ((o) != (void*)0xdeadbeaf && (((o)->typePtr != NULL) ? ((o)->typePtr->name != NULL) : ((o)->bytes != NULL)) && (((o)->bytes != NULL) ? NSF_PLAUSIBLE_LENGTH(o) : 1) && NSF_PLAUSIBLE_REFCOUNT(o))
 #else
 # define ISOBJ(o)
 #endif
@@ -373,10 +381,10 @@
 
 #ifdef USE_TCL_STUBS
 # define DECR_REF_COUNT(A) \
-  MEM_COUNT_FREE("INCR_REF_COUNT" #A,(A)); assert((A)->refCount >= 0);	\
+  MEM_COUNT_FREE("INCR_REF_COUNT" #A,(A)); assert(NSF_PLAUSIBLE_REFCOUNT(A)); \
   Tcl_DecrRefCount(A)
 # define DECR_REF_COUNT2(name,A)					\
-  MEM_COUNT_FREE("INCR_REF_COUNT-" name,(A)); assert((A)->refCount >= 0); \
+  MEM_COUNT_FREE("INCR_REF_COUNT-" name,(A)); assert(NSF_PLAUSIBLE_REFCOUNT(A)); \
   Tcl_DecrRefCount(A)
 #else
 # define DECR_REF_COUNT(A) \