diff --git a/mod/lesson/locallib.php b/mod/lesson/locallib.php
index 8d50af0f459..0d089adbaf1 100644
--- a/mod/lesson/locallib.php
+++ b/mod/lesson/locallib.php
@@ -323,7 +323,7 @@ function lesson_grade($lesson, $ntries, $userid = 0) {
                 $attempt = end($attempts);
                 // If essay question, handle it, otherwise add to score
                 if ($page->requires_manual_grading()) {
-                    $useranswerobj = unserialize($attempt->useranswer);
+                    $useranswerobj = unserialize_object($attempt->useranswer);
                     if (isset($useranswerobj->score)) {
                         $earned += $useranswerobj->score;
                     }
@@ -2919,11 +2919,11 @@ class lesson extends lesson_base {
 
         if ($dependentlesson = $DB->get_record('lesson', array('id' => $this->properties->dependency))) {
             // Lesson exists, so we can proceed.
-            $conditions = unserialize($this->properties->conditions);
+            $conditions = unserialize_object($this->properties->conditions);
             // Assume false for all.
             $errors = array();
             // Check for the timespent condition.
-            if ($conditions->timespent) {
+            if (!empty($conditions->timespent)) {
                 $timespent = false;
                 if ($attempttimes = $DB->get_records('lesson_timer', array("userid" => $USER->id, "lessonid" => $dependentlesson->id))) {
                     // Go through all the times and test to see if any of them satisfy the condition.
@@ -2939,7 +2939,7 @@ class lesson extends lesson_base {
                 }
             }
             // Check for the gradebetterthan condition.
-            if ($conditions->gradebetterthan) {
+            if (!empty($conditions->gradebetterthan)) {
                 $gradebetterthan = false;
                 if ($studentgrades = $DB->get_records('lesson_grades', array("userid" => $USER->id, "lessonid" => $dependentlesson->id))) {
                     // Go through all the grades and test to see if any of them satisfy the condition.
@@ -2954,7 +2954,7 @@ class lesson extends lesson_base {
                 }
             }
             // Check for the completed condition.
-            if ($conditions->completed) {
+            if (!empty($conditions->completed)) {
                 if (!$DB->count_records('lesson_grades', array('userid' => $USER->id, 'lessonid' => $dependentlesson->id))) {
                     $errors[] = get_string('completederror', 'lesson');
                 }
diff --git a/mod/lesson/mod_form.php b/mod/lesson/mod_form.php
index be2cca26a53..e2a4e404e06 100644
--- a/mod/lesson/mod_form.php
+++ b/mod/lesson/mod_form.php
@@ -358,10 +358,10 @@ class mod_lesson_mod_form extends moodleform_mod {
      **/
     public function data_preprocessing(&$defaultvalues) {
         if (isset($defaultvalues['conditions'])) {
-            $conditions = unserialize($defaultvalues['conditions']);
-            $defaultvalues['timespent'] = $conditions->timespent;
-            $defaultvalues['completed'] = $conditions->completed;
-            $defaultvalues['gradebetterthan'] = $conditions->gradebetterthan;
+            $conditions = unserialize_object($defaultvalues['conditions']);
+            $defaultvalues['timespent'] = $conditions->timespent ?? 0;
+            $defaultvalues['completed'] = !empty($conditions->completed);
+            $defaultvalues['gradebetterthan'] = $conditions->gradebetterthan ?? 0;
         }
 
         // Set up the completion checkbox which is not part of standard data.
diff --git a/mod/lesson/pagetypes/essay.php b/mod/lesson/pagetypes/essay.php
index 41b3d6fada8..0cde105b50c 100644
--- a/mod/lesson/pagetypes/essay.php
+++ b/mod/lesson/pagetypes/essay.php
@@ -56,9 +56,9 @@ class lesson_page_type_essay extends lesson_page {
      * @return object
      */
     static public function extract_useranswer($useranswer) {
-        $essayinfo = unserialize($useranswer);
+        $essayinfo = unserialize_object($useranswer);
         if (!isset($essayinfo->responseformat)) {
-            $essayinfo->response = text_to_html($essayinfo->response, false, false);
+            $essayinfo->response = text_to_html($essayinfo->response ?? '', false, false);
             $essayinfo->responseformat = FORMAT_HTML;
         }
         return $essayinfo;
@@ -150,7 +150,7 @@ class lesson_page_type_essay extends lesson_page {
                 $editoroptions['context'], 'mod_lesson', 'essay_answers', $attempt->id);
 
             // Update the student response to have the modified link.
-            $useranswer = unserialize($attempt->useranswer);
+            $useranswer = unserialize_object($attempt->useranswer);
             $useranswer->answer = $formdata->answer;
             $useranswer->answerformat = $formdata->answerformat;
             $attempt->useranswer = serialize($useranswer);