diff --git a/C4/AuthoritiesMarc.pm b/C4/AuthoritiesMarc.pm
index 4cbed9e95d..dfc0749861 100644
--- a/C4/AuthoritiesMarc.pm
+++ b/C4/AuthoritiesMarc.pm
@@ -1749,7 +1749,8 @@ sub merge {
                 if ( exists $controlled_ind->{sub2} ) {    # thesaurus info
                     if ( defined $controlled_ind->{sub2} ) {
 
-                        # Add or replace
+                        # Add or replace when not empty. If empty, an existing
+                        # $2 value is retained (see also bug 40119).
                         if ( $controlled_ind->{sub2} ne q{} ) {
                             $field_to->update( 2 => $controlled_ind->{sub2} );
                         }