Index: openacs-4/packages/xotcl-core/tcl/06-param-procs.tcl
===================================================================
RCS file: /usr/local/cvsroot/openacs-4/packages/xotcl-core/tcl/06-param-procs.tcl,v
diff -u -N -r1.24.2.8 -r1.24.2.9
--- openacs-4/packages/xotcl-core/tcl/06-param-procs.tcl	9 Jun 2020 12:45:20 -0000	1.24.2.8
+++ openacs-4/packages/xotcl-core/tcl/06-param-procs.tcl	10 Jun 2020 16:10:55 -0000	1.24.2.9
@@ -330,7 +330,9 @@
     }
     parameter instproc clear_per_package_instance_value {package_id} {
       set array [:per_package_id_name $package_id]
-      nsv_dict unset $array $package_id ${:parameter_name}
+      if {[nsv_dict exists $array $package_id ${:parameter_name}]} {
+        nsv_dict unset $array $package_id ${:parameter_name}
+      }
     }
     parameter instproc get {-package_id:required} {
       set array [:per_package_id_name $package_id]