Index: generic/nsf.c
===================================================================
diff -u -r4b9ddd1b06ce34fd352791fcae53ad91a79b43ce -r14101af0c3253e79cf68622cb05e51eb780e1f39
--- generic/nsf.c	(.../nsf.c)	(revision 4b9ddd1b06ce34fd352791fcae53ad91a79b43ce)
+++ generic/nsf.c	(.../nsf.c)	(revision 14101af0c3253e79cf68622cb05e51eb780e1f39)
@@ -19999,6 +19999,11 @@
     break;
 
   case AssertionsubcmdClass_invarIdx:
+
+    if (!NsfObjectIsClass(object)) {
+      return NsfPrintError(interp, "object is not a class");
+    }
+
     class = (NsfClass *)object;
     if (arg) {
       NsfClassOpt *opt = NsfRequireClassOpt(class);
@@ -21427,7 +21432,8 @@
 
   case CurrentoptionClassIdx: /* class subcommand */
     cscPtr = CallStackGetTopFrame0(interp);
-    Tcl_SetObjResult(interp, cscPtr->cl ? cscPtr->cl->object.cmdName : NsfGlobalObjs[NSF_EMPTY]);
+    Tcl_SetObjResult(interp, cscPtr && cscPtr->cl ? 
+		     cscPtr->cl->object.cmdName : NsfGlobalObjs[NSF_EMPTY]);
     break;
 
   case CurrentoptionActivelevelIdx:
@@ -21439,14 +21445,19 @@
     Tcl_Obj **nobjv;
 
     cscPtr = CallStackGetTopFrame(interp, &framePtr);
-    if (cscPtr->objv) {
-      nobjc = cscPtr->objc;
-      nobjv = (Tcl_Obj **)cscPtr->objv;
+
+    if (cscPtr) {
+      if (cscPtr->objv) {
+	nobjc = cscPtr->objc;
+	nobjv = (Tcl_Obj **)cscPtr->objv;
+      } else {
+	nobjc = Tcl_CallFrame_objc(framePtr);
+	nobjv = (Tcl_Obj **)Tcl_CallFrame_objv(framePtr);
+      }
+      Tcl_SetObjResult(interp, Tcl_NewListObj(nobjc-1, nobjv+1));
     } else {
-      nobjc = Tcl_CallFrame_objc(framePtr);
-      nobjv = (Tcl_Obj **)Tcl_CallFrame_objv(framePtr);
+      return NsfPrintError(interp,  "can't find proc");
     }
-    Tcl_SetObjResult(interp, Tcl_NewListObj(nobjc-1, nobjv+1));
     break;
   }