Index: library/nx/nx.tcl
===================================================================
diff -u -r7257200f41c3bd0d0309db6932e5788d860fa119 -rfe643dd6330cf3deb036335f20ce4113a2e2aee3
--- library/nx/nx.tcl	(.../nx.tcl)	(revision 7257200f41c3bd0d0309db6932e5788d860fa119)
+++ library/nx/nx.tcl	(.../nx.tcl)	(revision fe643dd6330cf3deb036335f20ce4113a2e2aee3)
@@ -1646,6 +1646,8 @@
 	  set name $spec
 	}
 	set :$name $value
+      } else {
+	error "Variable definition for '$name' (without default and accessor) is useless"
       }
       return
     }