diff --git a/mailpoet/lib/Automation/Engine/Data/WorkflowRunLog.php b/mailpoet/lib/Automation/Engine/Data/WorkflowRunLog.php
index 93f35b717f..7a3d680171 100644
--- a/mailpoet/lib/Automation/Engine/Data/WorkflowRunLog.php
+++ b/mailpoet/lib/Automation/Engine/Data/WorkflowRunLog.php
@@ -3,6 +3,9 @@
 namespace MailPoet\Automation\Engine\Data;
 
 use DateTimeImmutable;
+use InvalidArgumentException;
+use MailPoet\Automation\Engine\Exceptions\InvalidStateException;
+use MailPoet\Automation\Engine\Exceptions\UnexpectedValueException;
 use MailPoet\Automation\Engine\Utils\Json;
 use Throwable;
 
@@ -102,6 +105,17 @@ class WorkflowRunLog {
    * @return void
    */
   public function setData(string $key, $value): void {
+    try {
+      $newData = $this->getData();
+      $newData[$key] = $value;
+      $encoded = Json::encode($newData);
+      $decoded = Json::decode($encoded);
+      if ($decoded !== $newData) {
+        throw new InvalidArgumentException('$value must be serializable');
+      }
+    } catch (InvalidStateException | InvalidArgumentException | UnexpectedValueException $e) {
+      throw new InvalidArgumentException("Invalid data provided for key $key.");
+    }
     $this->data[$key] = $value;
   }
 
diff --git a/mailpoet/tests/integration/Automation/Engine/Data/WorkflowRunLogTest.php b/mailpoet/tests/integration/Automation/Engine/Data/WorkflowRunLogTest.php
index 7adbf6333f..5f80b7d9ee 100644
--- a/mailpoet/tests/integration/Automation/Engine/Data/WorkflowRunLogTest.php
+++ b/mailpoet/tests/integration/Automation/Engine/Data/WorkflowRunLogTest.php
@@ -58,6 +58,16 @@ class WorkflowRunLogTest extends \MailPoetTest {
     $this->assertSame('value', $data['key']);
   }
 
+  public function testItDoesNotAllowSettingDataThatCannotBeSaved(): void {
+    $log = new WorkflowRunLog(1, 'step-id', []);
+    $badData = [
+      function() { echo 'closures cannot be serialized'; }
+    ];
+    $this->expectException(\InvalidArgumentException::class);
+    $log->setData('badData', $badData);
+    expect($log->getData())->count(0);
+  }
+
   public function testItGetsExposedViaAction(): void {
     $this->wp->addAction(Hooks::WORKFLOW_RUN_LOG_AFTER_STEP_RUN, function(WorkflowRunLog $log) {
       $log->setData('test', 'value');