Index: generic/nsf.c
===================================================================
diff -u -r5163190ade6f6a2abcd8b7f0265f74457557a61c -r36b29a9f1f308d692dc1b4ce35c4adb0948bd6bd
--- generic/nsf.c	(.../nsf.c)	(revision 5163190ade6f6a2abcd8b7f0265f74457557a61c)
+++ generic/nsf.c	(.../nsf.c)	(revision 36b29a9f1f308d692dc1b4ce35c4adb0948bd6bd)
@@ -427,7 +427,6 @@
 static int MethodSourceMatches(int withSource, NsfClass *cl, NsfObject *object);
 
 static NsfObjectOpt *NsfRequireObjectOpt(NsfObject *object) nonnull(1) returns_nonnull;
-static NsfClassOpt * NsfRequireClassOpt(/*@notnull@*/ NsfClass *cl) nonnull(1) returns_nonnull;
 
 static int ObjectSystemsCheckSystemMethod(Tcl_Interp *interp, CONST char *methodName,
                                           NsfObject *object, unsigned int flags)
@@ -4148,7 +4147,7 @@
 }
 
 
-static NsfClassOpt *
+NsfClassOpt *
 NsfRequireClassOpt(/*@notnull@*/ NsfClass *cl) {
 
   assert(cl);
@@ -18724,16 +18723,24 @@
     }
   }
 
-  if (clopt) {
+  if (clopt != NULL) {
     /*
      *  Remove this class from all isClassMixinOf lists and clear the
      *  class mixin list
      */
-    if (clopt->classMixins) RemoveFromClassMixinsOf(clopt->id, clopt->classMixins);
+    if (clopt->classMixins) {
+      RemoveFromClassMixinsOf(clopt->id, clopt->classMixins);
+    }
 
     CmdListFree(&clopt->classMixins, GuardDel);
     CmdListFree(&clopt->classFilters, GuardDel);
 
+    if (clopt->mixinRegObjs != NULL) {
+      NsfMixinregInvalidate(interp, clopt->mixinRegObjs);
+      DECR_REF_COUNT2("mixinRegObjs", clopt->mixinRegObjs);
+      clopt->mixinRegObjs = NULL;
+    }
+
     if (!recreate) {
       /*
        *  Remove this class from all mixin lists and clear the isObjectMixinOf list
@@ -18805,9 +18812,9 @@
     MEM_COUNT_FREE("Tcl_InitHashTable", &cl->instances);
   }
 
-  if ((clopt) && (!recreate)) {
+  if (clopt != NULL && recreate == 0) {
     FREE(NsfClassOpt, clopt);
-    cl->opt = 0;
+    cl->opt = NULL;
   }
 
   if (subClasses) {
@@ -26770,8 +26777,8 @@
   int canFree;
 } NsfParamWrapper;
 
-static Tcl_DupInternalRepProc        ParamDupInteralRep;
-static Tcl_FreeInternalRepProc        ParamFreeInternalRep;
+static Tcl_DupInternalRepProc      ParamDupInteralRep;
+static Tcl_FreeInternalRepProc     ParamFreeInternalRep;
 static Tcl_UpdateStringProc        ParamUpdateString;
 
 static void ParamUpdateString(Tcl_Obj *objPtr) nonnull(1);
Index: generic/nsfInt.h
===================================================================
diff -u -r15beef03245b7fe339e4bd22cc8ee5da92d789a6 -r36b29a9f1f308d692dc1b4ce35c4adb0948bd6bd
--- generic/nsfInt.h	(.../nsfInt.h)	(revision 15beef03245b7fe339e4bd22cc8ee5da92d789a6)
+++ generic/nsfInt.h	(.../nsfInt.h)	(revision 36b29a9f1f308d692dc1b4ce35c4adb0948bd6bd)
@@ -564,6 +564,7 @@
   NsfCmdList *isObjectMixinOf;
   NsfCmdList *isClassMixinOf;
   NsfAssertionStore *assertions;
+  Tcl_Obj *mixinRegObjs;
 #ifdef NSF_OBJECTDATA
   Tcl_HashTable *objectdata;
 #endif
@@ -716,13 +717,18 @@
 
 /* obj types */
 EXTERN Tcl_ObjType NsfMixinregObjType;
-int NsfMixinregGet(Tcl_Interp *interp, Tcl_Obj *obj, NsfClass **clPtr, Tcl_Obj **guardObj)
+EXTERN int NsfMixinregGet(Tcl_Interp *interp, Tcl_Obj *obj, NsfClass **clPtr, Tcl_Obj **guardObj)
   nonnull(1) nonnull(2) nonnull(3) nonnull(4);
+EXTERN int NsfMixinregInvalidate(Tcl_Interp *interp, Tcl_Obj *obj) 
+  nonnull(1) nonnull(2);
 
 EXTERN Tcl_ObjType NsfFilterregObjType;
-int NsfFilterregGet(Tcl_Interp *interp, Tcl_Obj *obj, Tcl_Obj **filterObj, Tcl_Obj **guardObj)
+EXTERN int NsfFilterregGet(Tcl_Interp *interp, Tcl_Obj *obj, Tcl_Obj **filterObj, Tcl_Obj **guardObj)
   nonnull(1) nonnull(2) nonnull(3) nonnull(4);
 
+EXTERN NsfClassOpt *NsfRequireClassOpt(/*@notnull@*/ NsfClass *cl) nonnull(1) returns_nonnull;
+
+
 /* Next Scripting ShadowTclCommands */
 typedef struct NsfShadowTclCommandInfo {
   TclObjCmdProcType proc;
Index: generic/nsfObj.c
===================================================================
diff -u -r5163190ade6f6a2abcd8b7f0265f74457557a61c -r36b29a9f1f308d692dc1b4ce35c4adb0948bd6bd
--- generic/nsfObj.c	(.../nsfObj.c)	(revision 5163190ade6f6a2abcd8b7f0265f74457557a61c)
+++ generic/nsfObj.c	(.../nsfObj.c)	(revision 36b29a9f1f308d692dc1b4ce35c4adb0948bd6bd)
@@ -350,7 +350,7 @@
 {
   Mixinreg *mixinRegPtr = (Mixinreg *)objPtr->internalRep.twoPtrValue.ptr1;
 
-  assert(mixinRegPtr);
+  assert(mixinRegPtr != NULL);
   /*fprintf(stderr, "MixinregFreeInternalRep freeing mixinReg %p class %p guard %p refcount before decr %d\n",
     mixinRegPtr, mixinRegPtr->mixin, mixinRegPtr->guardObj, (&(mixinRegPtr->mixin)->object)->refCount);*/
 
@@ -365,6 +365,8 @@
    * ... and free structure
    */
   FREE(Mixinreg, mixinRegPtr);
+  objPtr->internalRep.twoPtrValue.ptr1 = NULL;
+  objPtr->typePtr = NULL;
 }
 
 /*
@@ -405,8 +407,8 @@
  */
 static int
 MixinregSetFromAny(
-    Tcl_Interp *interp,		/* Used for error reporting if not NULL. */
-    register Tcl_Obj *objPtr)	/* The object to convert. */
+    Tcl_Interp *interp,	/* Used for error reporting if not NULL. */
+    Tcl_Obj *objPtr)	/* The object to convert. */
 {
   NsfClass *mixin = NULL;
   Tcl_Obj *guardObj = NULL, *nameObj = NULL;
@@ -455,6 +457,18 @@
   NsfObjectRefCountIncr((&mixin->object));
   if (guardObj) {INCR_REF_COUNT2("mixinRegPtr->guardObj", guardObj);}
 
+  /*
+   * Build list of tcl-objs per mixin class for invalidation.
+   */
+  { NsfClassOpt *clOpt = NsfRequireClassOpt(mixin);
+    if (clOpt->mixinRegObjs == NULL) {
+      clOpt->mixinRegObjs = Tcl_NewListObj(1, &objPtr);
+      INCR_REF_COUNT2("mixinRegObjs", clOpt->mixinRegObjs);
+    } else {
+      Tcl_ListObjAppendElement(interp, clOpt->mixinRegObjs, objPtr);
+    }
+  }
+
   /*fprintf(stderr, "MixinregSetFromAny alloc mixinReg %p class %p guard %p object->refCount %d\n",
     mixinRegPtr, mixinRegPtr->mixin, mixinRegPtr->guardObj, ((&mixin->object)->refCount));*/
 
@@ -507,7 +521,8 @@
       /*
        * The cmd is deleted. retry to refetch it.
        */
-      /*fprintf(stderr, "... we have to refetch \n");*/
+      /*fprintf(stderr, "### we have to refetch internal rep of obj %p refCount %d\n",
+        obj, obj->refCount);*/
 
       if (MixinregSetFromAny(interp, obj) == TCL_OK) {
 	mixinRegPtr = obj->internalRep.twoPtrValue.ptr1;
@@ -528,6 +543,41 @@
 /*
  *----------------------------------------------------------------------
  *
+ * NsfMixinregInvalidate --
+ *
+ *      MixinClasses keep a list of Tcl_Objs of type mixinReg.
+ *      When a class is deleted, this call makes sure, that
+ *      non-of these have th chance to point to a stale entry.
+ *
+ * Results:
+ *      Tcl result code
+ *
+ * Side effects:
+ *      Freeing internal rep of Tcl_Objs.
+ *
+ *----------------------------------------------------------------------
+ */
+
+int
+NsfMixinregInvalidate(Tcl_Interp *interp, Tcl_Obj *obj) {
+  int i, result, oc = 0;
+  Tcl_Obj **objv;
+
+  result = Tcl_ListObjGetElements(interp, obj, &oc, &objv);
+
+  for (i= 0; i < oc; i++) {
+    if (objv[i]->typePtr == &NsfMixinregObjType) {
+      MixinregFreeInternalRep((objv[i]));
+    }
+  }
+
+  return result;
+}
+
+
+/*
+ *----------------------------------------------------------------------
+ *
  *  Filterreg Tcl_Obj type --
  *
  *      The filter registration type is an Tcl_Obj type carrying a the
@@ -584,6 +634,8 @@
    * ... and free structure
    */
   FREE(Filterreg, filterregPtr);
+  objPtr->internalRep.twoPtrValue.ptr1 = NULL;
+  objPtr->typePtr = NULL;
 }
 
 /*