Index: generic/gentclAPI.tcl
===================================================================
diff -u -r0e247f618346f3d25ffe2c30633bf75bc1f67055 -rec070e95f0d54ebae32e27b9a3251d7ec05d39ad
--- generic/gentclAPI.tcl	(.../gentclAPI.tcl)	(revision 0e247f618346f3d25ffe2c30633bf75bc1f67055)
+++ generic/gentclAPI.tcl	(.../gentclAPI.tcl)	(revision ec070e95f0d54ebae32e27b9a3251d7ec05d39ad)
@@ -156,7 +156,8 @@
       set argNum 3
       append intro \
           "  NsfClass *cl =  NsfObjectToClass(clientData);" \n\n \
-          "  assert(objc > 0);" \n
+          "  assert(objc > 0);" \n \
+          "  if (unlikely(cl == NULL)) return NsfDispatchClientDataError(interp, clientData, \"class\", ObjStr(objv\[0\]));"
     }
     object {
       set a  [list obj]