Index: TODO
===================================================================
diff -u -r35e0efc0d8851a10c70071c12112b11538ce97af -raa1943604c4de3d538dff67d5e9e238323d37474
--- TODO	(.../TODO)	(revision 35e0efc0d8851a10c70071c12112b11538ce97af)
+++ TODO	(.../TODO)	(revision aa1943604c4de3d538dff67d5e9e238323d37474)
@@ -813,8 +813,10 @@
   to xotcl.c, aligned order of method definitions
 
 - removed "[o exists varname]" from next scripting language
-
 - reanimated "info vars" to show locals in eval method
+- provide error messages for [objectproperty ... type ...]
+- replace 0 by NULL in calls to GetClassFromObj()
+- extended regression test
 
 TODO: 
 - rename source files from xotcl{Int}.{ch}->next*.* | next-scripting*.* ?
Index: generic/predefined.h
===================================================================
diff -u -r35e0efc0d8851a10c70071c12112b11538ce97af -raa1943604c4de3d538dff67d5e9e238323d37474
--- generic/predefined.h	(.../predefined.h)	(revision 35e0efc0d8851a10c70071c12112b11538ce97af)
+++ generic/predefined.h	(.../predefined.h)	(revision aa1943604c4de3d538dff67d5e9e238323d37474)
@@ -315,7 +315,8 @@
 "proc ::nx::core::parametersFromSlots {obj} {\n"
 "set parameterdefinitions [list]\n"
 "foreach slot [::nx::objectInfo slotobjects $obj] {\n"
-"if {[::nx::core::objectproperty $obj type ::xotcl::Object] &&\n"
+"if {[::nx::core::objectproperty ::xotcl::Object class]\n"
+"&& [::nx::core::objectproperty $obj type ::xotcl::Object] &&\n"
 "([$slot name] eq \"mixin\" || [$slot name] eq \"filter\")} continue\n"
 "array set \"\" [$slot toParameterSyntax]\n"
 "lappend parameterdefinitions -$(oparam)}\n"
Index: generic/predefined.xotcl
===================================================================
diff -u -r35e0efc0d8851a10c70071c12112b11538ce97af -raa1943604c4de3d538dff67d5e9e238323d37474
--- generic/predefined.xotcl	(.../predefined.xotcl)	(revision 35e0efc0d8851a10c70071c12112b11538ce97af)
+++ generic/predefined.xotcl	(.../predefined.xotcl)	(revision aa1943604c4de3d538dff67d5e9e238323d37474)
@@ -784,9 +784,10 @@
     foreach slot [::nx::objectInfo slotobjects $obj] {
       # Skip some slots for xotcl; 
       # TODO: maybe different parameterFromSlots for xotcl?
-      if {[::nx::core::objectproperty $obj type ::xotcl::Object] && 
+      if {[::nx::core::objectproperty ::xotcl::Object class] 
+	  && [::nx::core::objectproperty $obj type ::xotcl::Object] && 
           ([$slot name] eq "mixin" || [$slot name] eq "filter")
-        } continue
+      } continue
       array set "" [$slot toParameterSyntax]
       lappend parameterdefinitions -$(oparam)
     }
Index: generic/xotcl.c
===================================================================
diff -u -ra2a10538733f58248a38ab9d13d342ebd0fb475d -raa1943604c4de3d538dff67d5e9e238323d37474
--- generic/xotcl.c	(.../xotcl.c)	(revision a2a10538733f58248a38ab9d13d342ebd0fb475d)
+++ generic/xotcl.c	(.../xotcl.c)	(revision aa1943604c4de3d538dff67d5e9e238323d37474)
@@ -644,7 +644,7 @@
       result = callMethod((ClientData) baseClass, interp, methodObj,
                           3, &nameObj, XOTCL_CM_NO_PROTECT);
       if (result == TCL_OK) {
-        result = GetClassFromObj(interp, objPtr, cl, 0);
+        result = GetClassFromObj(interp, objPtr, cl, NULL);
       }
     }
     DECR_REF_COUNT(nameObj);
@@ -6227,7 +6227,7 @@
   if (pPtr->converterArg == NULL) 
     return TCL_OK;
 
-  if ((GetClassFromObj(interp, pPtr->converterArg, &cl, 0) == TCL_OK)
+  if ((GetClassFromObj(interp, pPtr->converterArg, &cl, NULL) == TCL_OK)
       && isSubType(object->cl, cl)) {
     return TCL_OK;
   }
@@ -6254,7 +6254,7 @@
 static int convertToClass(Tcl_Interp *interp, Tcl_Obj *objPtr,  XOTclParam CONST *pPtr, 
 			  ClientData *clientData, Tcl_Obj **outObjPtr) {
   *outObjPtr = objPtr;
-  if (GetClassFromObj(interp, objPtr, (XOTclClass **)clientData, 0) == TCL_OK) {
+  if (GetClassFromObj(interp, objPtr, (XOTclClass **)clientData, NULL) == TCL_OK) {
     return objectOfType(interp, (XOTclObject *)*clientData, "class", objPtr, pPtr);
   }
   return XOTclObjErrType(interp, objPtr, "class", pPtr->name);
@@ -7830,7 +7830,7 @@
 
     if (result == TCL_OK) {
       Tcl_Obj *nameObj = Tcl_GetObjResult(interp);
-      if (GetClassFromObj(interp, nameObj, &defaultClass, 0) != TCL_OK) {
+      if (GetClassFromObj(interp, nameObj, &defaultClass, NULL) != TCL_OK) {
 	XOTclErrMsg(interp, "default superclass is not a class", TCL_STATIC);
       }
       /*fprintf(stderr, "DefaultSuperClass for %s got from var %s\n", className(cl), ObjStr(nameObj));*/
@@ -11170,7 +11170,7 @@
   if (isTypeString(constraintString)) {
     if (arg== NULL) return XOTclObjErrArgCnt(interp, NULL, NULL, "type <object> <type>");
     success = (GetObjectFromObj(interp, valueObj, &object) == TCL_OK)
-      && (GetClassFromObj(interp, arg, &typeClass, 0) == TCL_OK)
+      && (GetClassFromObj(interp, arg, &typeClass, NULL) == TCL_OK)
       && isSubType(object->cl, typeClass);
 
     Tcl_SetIntObj(Tcl_GetObjResult(interp), success);
@@ -11182,14 +11182,14 @@
     if (*constraintString == 'o') {
       success = (GetObjectFromObj(interp, valueObj, &object) == TCL_OK);
     } else {
-      success = (GetClassFromObj(interp, valueObj, (XOTclClass **)&object, 0) == TCL_OK);
+      success = (GetClassFromObj(interp, valueObj, (XOTclClass **)&object, NULL) == TCL_OK);
     }
     if (success && withType) {
-      success = (GetClassFromObj(interp, withType, &typeClass, 0) == TCL_OK)
+      success = (GetClassFromObj(interp, withType, &typeClass, NULL) == TCL_OK)
         && isSubType(object->cl, typeClass);
     }
     if (success && withHasmixin) {
-      success = (GetClassFromObj(interp, withHasmixin, &mixinClass, 0) == TCL_OK)
+      success = (GetClassFromObj(interp, withHasmixin, &mixinClass, NULL) == TCL_OK)
         && hasMixin(interp, object, mixinClass);
     }
     Tcl_SetIntObj(Tcl_GetObjResult(interp), success);
@@ -11699,9 +11699,13 @@
   switch (objectkind) {
   case ObjectkindTypeIdx:
     if (valueObj == NULL) return XOTclObjErrArgCnt(interp, NULL, NULL, "<object> type <type>");
-    success = (GetObjectFromObj(interp, obj, &object) == TCL_OK)
-      && (GetClassFromObj(interp, valueObj, &cl, 0) == TCL_OK)
-      && isSubType(object->cl, cl);
+    if (GetObjectFromObj(interp, obj, &object) != TCL_OK) {
+      return XOTclObjErrType(interp, obj, "object", "object");
+    }
+    if (GetClassFromObj(interp, valueObj, &cl, NULL) != TCL_OK) {
+	return XOTclObjErrType(interp, valueObj, "class", "type");
+    }
+    success = isSubType(object->cl, cl);
     break;
 
   case ObjectkindObjectIdx:
@@ -11731,7 +11735,7 @@
   case ObjectkindHasmixinIdx:
     if (valueObj == NULL) return XOTclObjErrArgCnt(interp, NULL, NULL, "<object> hasmixin <class>");
     success = (GetObjectFromObj(interp, obj, &object) == TCL_OK)
-      && (GetClassFromObj(interp, valueObj, &cl, 0) == TCL_OK)
+      && (GetClassFromObj(interp, valueObj, &cl, NULL) == TCL_OK)
       && hasMixin(interp, object, cl);
     break;
   }
@@ -11848,7 +11852,7 @@
       return XOTclVarErrMsg(interp, "metaclass must be specified as third argument",
                             (char *) NULL);
     }
-    GetClassFromObj(interp, valueObj, &metaClass, 0);
+    GetClassFromObj(interp, valueObj, &metaClass, NULL);
     if (!metaClass) return XOTclObjErrType(interp, valueObj, "class", "");
 
     cl->object.flags |= XOTCL_IS_ROOT_CLASS;
Index: tests/object-system.xotcl
===================================================================
diff -u -r5670d611979156a6f4a6654fedc35e9e802e3dee -raa1943604c4de3d538dff67d5e9e238323d37474
--- tests/object-system.xotcl	(.../object-system.xotcl)	(revision 5670d611979156a6f4a6654fedc35e9e802e3dee)
+++ tests/object-system.xotcl	(.../object-system.xotcl)	(revision aa1943604c4de3d538dff67d5e9e238323d37474)
@@ -188,4 +188,12 @@
 
 ? {::nx::core::objectproperty ::C class} 0
 
+::nx::Class create ::C
+
+? {catch {::nx::core::objectproperty ::C type ::UNKNOWN}} 1
+? {catch {::C info is type ::xyz::Bar}} 1
+? {catch {::nx::core::objectproperty ::CCCC type ::nx::Object}} 1
+
+::C destroy
+
 puts stderr ===EXIT