Index: openacs-4/packages/adserver/tcl/adserver-procs.tcl
===================================================================
RCS file: /usr/local/cvsroot/openacs-4/packages/adserver/tcl/adserver-procs.tcl,v
diff -u -r1.6 -r1.7
--- openacs-4/packages/adserver/tcl/adserver-procs.tcl	29 Mar 2018 15:03:00 -0000	1.6
+++ openacs-4/packages/adserver/tcl/adserver-procs.tcl	12 Feb 2019 18:00:04 -0000	1.7
@@ -278,7 +278,7 @@
             } [adserver_cache_refresh]]} {
                 set user_id [ad_get_user_id]
                 if {$user_id == 0} {
-                    set user_id [db_null]
+                    set user_id ""
                 }
                 # ignore logged out clicks, for now....
                 db_dml adserver_defs_adv_user_insert {