diff --git a/lib/authlib.php b/lib/authlib.php
index 16e06f7580c..09f7b684d6e 100644
--- a/lib/authlib.php
+++ b/lib/authlib.php
@@ -1113,7 +1113,9 @@ function signup_validate_data($data, $files) {
 
     // Construct fake user object to check password policy against required information.
     $tempuser = new stdClass();
-    $tempuser->id = 1;
+    // To prevent errors with check_password_policy(),
+    // the temporary user and the guest must not share the same ID.
+    $tempuser->id = (int)$CFG->siteguest + 1;
     $tempuser->username = $data['username'];
     $tempuser->firstname = $data['firstname'];
     $tempuser->lastname = $data['lastname'];
diff --git a/lib/moodlelib.php b/lib/moodlelib.php
index a9c0177bff1..b59f74a7733 100644
--- a/lib/moodlelib.php
+++ b/lib/moodlelib.php
@@ -4966,7 +4966,7 @@ function get_complete_user_data($field, $value, $mnethostid = null, $throwexcept
 function check_password_policy($password, &$errmsg, $user = null) {
     global $CFG;
 
-    if (!empty($CFG->passwordpolicy)) {
+    if (!empty($CFG->passwordpolicy) && !isguestuser($user)) {
         $errmsg = '';
         if (core_text::strlen($password) < $CFG->minpasswordlength) {
             $errmsg .= '<div>'. get_string('errorminpasswordlength', 'auth', $CFG->minpasswordlength) .'</div>';