diff --git a/koha-tmpl/opac-tmpl/bootstrap/en/includes/masthead.inc b/koha-tmpl/opac-tmpl/bootstrap/en/includes/masthead.inc
index 07703d5f69..d17f893ebf 100644
--- a/koha-tmpl/opac-tmpl/bootstrap/en/includes/masthead.inc
+++ b/koha-tmpl/opac-tmpl/bootstrap/en/includes/masthead.inc
@@ -71,7 +71,12 @@
                         <div id="members">
                             <ul class="nav pull-right">
                                 [% UNLESS ( loggedinusername ) %]
-                                    <li><a href="/cgi-bin/koha/opac-user.pl" class="loginModal-trigger" role="button" data-toggle="modal">Log in to your account</a></li>
+                                    [% IF Koha.Preference('casAuthentication') %]
+                                        [%# CAS authentication is too complicated for modal window %]
+                                        <li><a href="/cgi-bin/koha/opac-user.pl">Log in to your account</a></li>
+                                    [% ELSE %]
+                                        <li><a href="/cgi-bin/koha/opac-user.pl" class="loginModal-trigger" role="button" data-toggle="modal">Log in to your account</a></li>
+                                    [% END %]
                                 [% END %]
                                 [% IF ( loggedinusername ) %]
                                     <li><p class="members navbar-text">Welcome, <a href="/cgi-bin/koha/opac-user.pl"><span class="loggedinusername">[% FOREACH USER_INF IN USER_INFO %][% USER_INF.title %] [% USER_INF.firstname %] [% USER_INF.surname %][% END %]</span></a></p></li>