diff --git a/src/lib/Sympa/Spindle/TransformIncoming.pm b/src/lib/Sympa/Spindle/TransformIncoming.pm
index f88253378..d2ee996bd 100644
--- a/src/lib/Sympa/Spindle/TransformIncoming.pm
+++ b/src/lib/Sympa/Spindle/TransformIncoming.pm
@@ -166,6 +166,7 @@ sub _twist {
             my $subject_field;
             my $re_regexp = Sympa::Regexps::re();
 
+            '' =~ /()/;    # clear $1
             if ($message->{'subject_charset'}) {
                 # Note that Unicode case-ignore match is performed.
                 my $uStem =