diff --git a/C4/Letters.pm b/C4/Letters.pm
index 3268f4ba80..1fe54a7c23 100644
--- a/C4/Letters.pm
+++ b/C4/Letters.pm
@@ -776,14 +776,18 @@ sub _parseletter_sth {
     parameters :
     - $letter : a hash to letter fields (title & content useful)
     - $table : the Koha table to parse.
-    - $values : table record hashref
+    - $values_in : table record hashref
     parse all fields from a table, and replace values in title & content with the appropriate value
     (not exported sub, used only internally)
 
 =cut
 
 sub _parseletter {
-    my ( $letter, $table, $values ) = @_;
+    my ( $letter, $table, $values_in ) = @_;
+
+    # Work on a local copy of $values_in (passed by reference) to avoid side effects
+    # in callers ( by changing / formatting values )
+    my $values = { %$values_in };
 
     if ( $table eq 'reserves' && $values->{'waitingdate'} ) {
         my @waitingdate = split /-/, $values->{'waitingdate'};