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 -N -r1.284.2.63 -r1.284.2.64
--- openacs-4/packages/xowiki/tcl/form-field-procs.tcl	13 Dec 2019 17:35:33 -0000	1.284.2.63
+++ openacs-4/packages/xowiki/tcl/form-field-procs.tcl	13 Dec 2019 18:14:28 -0000	1.284.2.64
@@ -1962,9 +1962,13 @@
 
   correct_when instproc check=valid_predicate {value} {
     set predicate [lindex $value 0]
-    set valid [expr {[:info methods answer_check=$predicate] ne ""}]
-    if {!$valid} {
-      :uplevel [list set errorMsg "invalid predicate $predicate"]
+    if {$predicate ne ""} {
+      set valid [expr {[:info methods answer_check=$predicate] ne ""}]
+      if {!$valid} {
+        :uplevel [list set errorMsg "invalid predicate $predicate"]
+      }
+    } else {
+      set valid 1
     }
     return $valid
   }