diff --git a/src/AddToCampaignHandler.php b/src/AddToCampaignHandler.php
index 7c03f6d6..746fe853 100644
--- a/src/AddToCampaignHandler.php
+++ b/src/AddToCampaignHandler.php
@@ -134,7 +134,7 @@ protected function getAvailableChangeSets()
      */
     protected function getInChangeSets($object)
     {
-        $inChangeSetIDs = array_unique(ChangeSetItem::get_for_object($object)->column('ChangeSetID'));
+        $inChangeSetIDs = array_unique(ChangeSetItem::get_for_object($object)->column('ChangeSetID') ?: []);
         if ($inChangeSetIDs > 0) {
             $changeSets = $this->getAvailableChangeSets()->filter([
                 'ID' => $inChangeSetIDs,
diff --git a/src/CampaignAdmin.php b/src/CampaignAdmin.php
index 7686cd12..40910ef5 100644
--- a/src/CampaignAdmin.php
+++ b/src/CampaignAdmin.php
@@ -205,7 +205,7 @@ protected function getPlaceholderGroups()
         $classes = Config::inst()->get(ChangeSet::class, 'important_classes');
 
         foreach ($classes as $class) {
-            if (!class_exists($class)) {
+            if (!class_exists((string) $class)) {
                 continue;
             }
             /** @var DataObject $item */
@@ -441,7 +441,7 @@ public function readCampaign(HTTPRequest $request)
         $accepts = $request->getAcceptMimetypes();
 
         //accept 'text/json' for legacy reasons
-        if (in_array('application/json', $accepts) || in_array('text/json', $accepts)) {
+        if (in_array('application/json', $accepts ?: []) || in_array('text/json', $accepts ?: [])) {
             $response->addHeader('Content-Type', 'application/json');
             if (!$request->param('Name')) {
                 return (new HTTPResponse(null, 400));
diff --git a/tests/behat/src/FixtureContext.php b/tests/behat/src/FixtureContext.php
index e332c677..623e6f32 100644
--- a/tests/behat/src/FixtureContext.php
+++ b/tests/behat/src/FixtureContext.php
@@ -180,7 +180,7 @@ public function iClickTheGalleryItem($name)
     public function iSeeTheCampaignItem($negate, $name)
     {
         $item = $this->getCampaignItem($name);
-        $shouldSee = !trim($negate);
+        $shouldSee = !trim((string) $negate);
 
         if ($shouldSee) {
             Assert::assertNotNull($item, sprintf('Item "%s" could not be found', $name));
@@ -219,7 +219,7 @@ public function stepCreateCampaignWithItems($id, $data)
 
         preg_match_all(
             '/"(?<content_class>[^"]+)"\s*=\s*"(?<identifier>[^"]+)"/',
-            $data,
+            (string) $data,
             $matches
         );
         $itemMap = array_combine($matches['content_class'], $matches['identifier']);