Index: openacs-4/packages/categories/tcl/categories-procs.tcl
===================================================================
RCS file: /usr/local/cvsroot/openacs-4/packages/categories/tcl/categories-procs.tcl,v
diff -u -r1.38 -r1.39
--- openacs-4/packages/categories/tcl/categories-procs.tcl	18 Sep 2018 18:28:55 -0000	1.38
+++ openacs-4/packages/categories/tcl/categories-procs.tcl	12 Feb 2019 17:12:19 -0000	1.39
@@ -160,7 +160,7 @@
 ad_proc -public category::change_parent {
     -category_id:required
     -tree_id:required
-    {-parent_id [db_null]}
+    {-parent_id ""}
 } {
     Changes parent category of a category.
     @option category_id category_id whose parent should change.