Index: library/nx/nx.tcl
===================================================================
diff -u -N -ra7b9d926225de93177a59bc1f2a6daf5228923c3 -r36a0f29d6f1ea1fde0810d05a1fedf3602b865cc
--- library/nx/nx.tcl	(.../nx.tcl)	(revision a7b9d926225de93177a59bc1f2a6daf5228923c3)
+++ library/nx/nx.tcl	(.../nx.tcl)	(revision 36a0f29d6f1ea1fde0810d05a1fedf3602b865cc)
@@ -2319,6 +2319,9 @@
   }
 
   ::nx::VariableSlot public method value=delete {obj prop -nocomplain:switch value} {
+    if {![:isMultivalued]} {
+      return -code error "property $prop of [set :domain] is not multivalued"
+    }
     set old [::nsf::var::get $obj $prop]
     set p [lsearch -glob $old $value]
     if {$p > -1} {