Index: generic/nsf.c
===================================================================
diff -u -r0e705d6ad1629ad4c0c7b0a2c67b01f658597abd -r242d5780b82de4363cd774a29ff7296270cdf0d0
--- generic/nsf.c	(.../nsf.c)	(revision 0e705d6ad1629ad4c0c7b0a2c67b01f658597abd)
+++ generic/nsf.c	(.../nsf.c)	(revision 242d5780b82de4363cd774a29ff7296270cdf0d0)
@@ -457,6 +457,20 @@
   Tcl_DStringFree(dsPtr);
 }
 
+#if 0
+static char *
+NsfErrorInfo(Tcl_Interp *interp) {
+  Tcl_Obj *value;
+  assert(interp);
+
+  value = Tcl_GetVar2Ex(interp, "::errorInfo", NULL, TCL_GLOBAL_ONLY);
+  if (value) {
+    return ObjStr(value);
+  }
+  return NULL;
+}
+#endif
+
 /*
  *----------------------------------------------------------------------
  *
@@ -10677,6 +10691,7 @@
                               (Namespace *) nsPtr, "body of proc",
                               procName);
     *flagsPtr &= ~NSF_CSC_CALL_IS_COMPILE;
+
     return result;
   }
 }
@@ -13061,8 +13076,10 @@
 			       resolvedCmd, cscPtr, methodName, &validCscPtr);
 
     if (unlikely(result == TCL_ERROR)) {
-      /*fprintf(stderr, "Call ErrInProc cl = %p, cmd %p, flags %.6x\n",
-	cl, cl ? cl->object.id : NULL, cl ? cl->object.flags : 0);*/
+      /*fprintf(stderr, "Call ErrInProc cl = %p, cmd %p, methodName %s flags %.6x\n",
+              cl, cl ? cl->object.id : NULL, methodName,
+              cl ? cl->object.flags : 0);*/
+
       result = NsfErrInProc(interp, cmdName,
 			    cl && cl->object.teardown ? cl->object.cmdName : NULL,
 			    methodName);
@@ -13266,7 +13283,6 @@
        */
       result = TCL_OK;
     } else {
-      /*fprintf(stderr, "%s init dispatch\n", ObjectName(object));*/
       result = CallMethod(object, interp, methodObj,
 			  objc+2, objv, 
                           flags|NSF_CM_IGNORE_PERMISSIONS|NSF_CSC_IMMEDIATE);
@@ -14812,15 +14828,15 @@
 
   if ((paramPtr->slotObj || paramPtr->converter == ConvertViaCmd) && paramPtr->type) {
     CONST char *converterNameString;
-    Tcl_Obj *converterNameObj;
+    Tcl_Obj *converterNameObj, *slotObj;
     NsfObject *paramObject;
     Tcl_Command cmd;
     NsfClass *pcl = NULL;
 
-    result = GetObjectFromObj(interp, paramPtr->slotObj ? paramPtr->slotObj :
-			      NsfGlobalObjs[NSF_METHOD_PARAMETER_SLOT_OBJ],
-			      &paramObject);
+    slotObj = paramPtr->slotObj ? paramPtr->slotObj : NsfGlobalObjs[NSF_METHOD_PARAMETER_SLOT_OBJ];
+    result = GetObjectFromObj(interp, slotObj, &paramObject);
     if (unlikely(result != TCL_OK)) {
+      NsfPrintError(interp, "non-existing slot object \"%s\"", ObjStr(slotObj));
       goto param_error;
     }
     if (paramPtr->converterName == NULL) {
@@ -14900,6 +14916,17 @@
   ParamFree(paramPtr);
   paramPtr->name = NULL;
 
+#if !defined(NDEBUG)
+  /*
+   * Whenever we return a TCL_ERROR, we expect that the interp result contains
+   * an error message.
+   */
+  {
+    char *errStr = ObjStr(Tcl_GetObjResult(interp));
+    assert(*errStr != '\0');
+  }
+#endif
+
   return TCL_ERROR;
 }
 
@@ -14955,6 +14982,7 @@
     for (i = 0; i < argsc; i++, paramPtr++) {
       result = ParamParse(interp, procNameObj, argsv[i], allowedOptinons,
 			  paramPtr, &possibleUnknowns, &plainParams, &nrNonposArgs);
+
       if (result == TCL_OK && paramPtr->converter == ConvertToNothing && i < argsc-1) {
 	result = NsfPrintError(interp,
 			       "parameter option \"args\" invalid for parameter \"%s\"; only allowed for last parameter",
@@ -15353,11 +15381,12 @@
   /* Check, if we are allowed to redefine the method */
   result = CanRedefineCmd(interp, nsPtr, defObject, methodName);
   if (likely(result == TCL_OK)) {
-    /* Yes, so obtain an method parameter definitions */
+    /* Yes, we can! ...so obtain an method parameter definitions */
     result = ParamDefsParse(interp, nameObj, args,
 			    NSF_DISALLOWED_ARG_METHOD_PARAMETER, 0,
 			    &parsedParam);
   }
+
   if (unlikely(result != TCL_OK)) {
     return result;
   }
@@ -26584,7 +26613,7 @@
                   NSF_s_set_idx, methodObj);*/
 
 	  result = NsfCallMethodWithArgs(interp, (Nsf_Object *)slotObject,
-                                         methodObj ? methodObj : NsfGlobalObjs[NSF_SET],
+                                         methodObj ? methodObj : NsfGlobalObjs[NSF_SLOT_SET],
 					 object->cmdName, 3, ov, NSF_CSC_IMMEDIATE);
 	}
 	if (result != TCL_OK) {
@@ -26712,7 +26741,7 @@
       NSF_s_get_idx, methodObj);*/
 
     result = NsfCallMethodWithArgs(interp, (Nsf_Object *)slotObject,
-                                   methodObj ? methodObj : NsfGlobalObjs[NSF_GET],
+                                   methodObj ? methodObj : NsfGlobalObjs[NSF_SLOT_GET],
 				   object->cmdName, 2, ov, NSF_CSC_IMMEDIATE);
     goto cget_exit;
   }
Index: generic/nsfInt.h
===================================================================
diff -u -r0e705d6ad1629ad4c0c7b0a2c67b01f658597abd -r242d5780b82de4363cd774a29ff7296270cdf0d0
--- generic/nsfInt.h	(.../nsfInt.h)	(revision 0e705d6ad1629ad4c0c7b0a2c67b01f658597abd)
+++ generic/nsfInt.h	(.../nsfInt.h)	(revision 242d5780b82de4363cd774a29ff7296270cdf0d0)
@@ -630,8 +630,9 @@
   NSF_EMPTY, NSF_ZERO, NSF_ONE,
   /* methods called internally */
   NSF_CONFIGURE, NSF_INITIALIZE, NSF_GET_PARAMETER_SPEC,
+  NSF_SLOT_GET, NSF_SLOT_SET,
   /* var names */
-  NSF_AUTONAMES, NSF_DEFAULTMETACLASS, NSF_DEFAULTSUPERCLASS, 
+  NSF_AUTONAMES, NSF_DEFAULTMETACLASS, NSF_DEFAULTSUPERCLASS,
   NSF_ARRAY_INITCMD, NSF_ARRAY_CMD,
   NSF_ARRAY_ALIAS, NSF_ARRAY_PARAMETERSYNTAX, 
   NSF_POSITION, NSF_POSITIONAL, NSF_CONFIGURABLE, NSF_PARAMETERSPEC,
@@ -651,15 +652,16 @@
 EXTERN char *NsfGlobalStrings[];
 #else
 char *NsfGlobalStrings[] = {
-  "", "0", "1", 
+  "", "0", "1",
   /* methods called internally */
   "configure", "initialize", "getParameterSpec",
+  "value=get", "value=set",
   /* var names */
   "__autonames", "__default_metaclass", "__default_superclass", "__initcmd", "__cmd",
-  "::nsf::alias", "::nsf::parameter::syntax", 
+  "::nsf::alias", "::nsf::parameter::syntax",
   "position", "positional", "configurable", "parameterSpec",
   /* object/class names */
-  "::nx::methodParameterSlot", 
+  "::nx::methodParameterSlot",
   /* constants */
   "alias", "args", "cmd", "filter",  "forward", 
   "method", "object", "setter", "settername", "valuecheck",