Index: openacs-4/packages/xowiki/tcl/form-field-procs.tcl
===================================================================
RCS file: /usr/local/cvsroot/openacs-4/packages/xowiki/tcl/form-field-procs.tcl,v
diff -u -r1.284.2.149 -r1.284.2.150
--- openacs-4/packages/xowiki/tcl/form-field-procs.tcl	4 Mar 2021 18:54:05 -0000	1.284.2.149
+++ openacs-4/packages/xowiki/tcl/form-field-procs.tcl	6 Mar 2021 14:27:04 -0000	1.284.2.150
@@ -4020,11 +4020,16 @@
     if {[info exists :answer]} {
       set count 1
       set :answer_value {}
-      foreach a ${:answer} {
-        if {$a} {
-          lappend :answer_value $count
+      try {
+        foreach a ${:answer} {
+          if {$a} {
+            lappend :answer_value $count
+          }
+          incr count
         }
-        incr count
+      } on error {errorMsg} {
+        ns_log error "${:name}: invalid answer value provided '${:answer}': must be list of booleans"
+        error $errorMsg
       }
       #ns_log notice "???? answer ${:answer} -> ${:answer_value}"
     }