diff --git a/auth/cas/auth.php b/auth/cas/auth.php
index 33b303a0349..1a888207d93 100644
--- a/auth/cas/auth.php
+++ b/auth/cas/auth.php
@@ -179,11 +179,20 @@ class auth_plugin_cas extends auth_plugin_ldap {
         static $connected = false;
 
         if (!$connected) {
+            // Form the base URL of the server with just the protocol and hostname.
+            $serverurl = new moodle_url("/");
+            $servicebaseurl = $serverurl->get_scheme() ? $serverurl->get_scheme() . "://" : '';
+            $servicebaseurl .= $serverurl->get_host();
+            // Add the port if set.
+            $servicebaseurl .= $serverurl->get_port() ? ':' . $serverurl->get_port() : '';
+
             // Make sure phpCAS doesn't try to start a new PHP session when connecting to the CAS server.
             if ($this->config->proxycas) {
-                phpCAS::proxy($this->config->casversion, $this->config->hostname, (int) $this->config->port, $this->config->baseuri, false);
+                phpCAS::proxy($this->config->casversion, $this->config->hostname, (int) $this->config->port, $this->config->baseuri,
+                    $servicebaseurl, false);
             } else {
-                phpCAS::client($this->config->casversion, $this->config->hostname, (int) $this->config->port, $this->config->baseuri, false);
+                phpCAS::client($this->config->casversion, $this->config->hostname, (int) $this->config->port,
+                    $this->config->baseuri, $servicebaseurl, false);
             }
             // Some CAS installs require SSLv3 that should be explicitly set.
             if (!empty($this->config->curl_ssl_version)) {