diff --git a/src/Controller/PluginRevisionController.php b/src/Controller/PluginRevisionController.php
index 6205ae1..96a83e6 100644
--- a/src/Controller/PluginRevisionController.php
+++ b/src/Controller/PluginRevisionController.php
@@ -157,7 +157,7 @@ class PluginRevisionController extends ControllerBase {
     $build['controls']['diff_layout'] = [
       '#type' => 'item',
       '#title' => $this->t('Layout'),
-      '#wrapper_attributes' => ['class' => 'diff-controls__item'],
+      '#wrapper_attributes' => ['class' => ['diff-controls__item']],
       'filter' => $this->buildLayoutNavigation($entity, $left_revision->getRevisionId(), $right_revision->getRevisionId(), $filter),
     ];
 
@@ -246,7 +246,7 @@ class PluginRevisionController extends ControllerBase {
       $element = [
         '#type' => 'item',
         '#title' => $this->t('Navigation'),
-        '#wrapper_attributes' => ['class' => 'diff-navigation'],
+        '#wrapper_attributes' => ['class' => ['diff-navigation']],
       ];
       $i = 0;
       // Find the previous revision.
diff --git a/src/DiffLayoutBase.php b/src/DiffLayoutBase.php
index dcb20f0..6850b83 100644
--- a/src/DiffLayoutBase.php
+++ b/src/DiffLayoutBase.php
@@ -143,7 +143,7 @@ abstract class DiffLayoutBase extends PluginBase implements DiffLayoutInterface,
         'diff_revisions' => [
           '#type' => 'item',
           '#title' => $this->t('Comparing'),
-          '#wrapper_attributes' => ['class' => 'diff-revision'],
+          '#wrapper_attributes' => ['class' => ['diff-revision']],
           'items' => [
             '#prefix' => '<div class="diff-revision__items">',
             '#suffix' => '</div>',
diff --git a/src/Plugin/diff/Layout/SplitFieldsDiffLayout.php b/src/Plugin/diff/Layout/SplitFieldsDiffLayout.php
index 6316b0a..c30ba78 100644
--- a/src/Plugin/diff/Layout/SplitFieldsDiffLayout.php
+++ b/src/Plugin/diff/Layout/SplitFieldsDiffLayout.php
@@ -105,7 +105,7 @@ class SplitFieldsDiffLayout extends DiffLayoutBase {
     $build['controls']['filter'] = [
       '#type' => 'item',
       '#title' => $this->t('Filter'),
-      '#wrapper_attributes' => ['class' => 'diff-controls__item'],
+      '#wrapper_attributes' => ['class' => ['diff-controls__item']],
       'options' => $this->buildFilterNavigation($entity, $left_revision, $right_revision, 'split_fields', $active_filter),
     ];
 
diff --git a/src/Plugin/diff/Layout/UnifiedFieldsDiffLayout.php b/src/Plugin/diff/Layout/UnifiedFieldsDiffLayout.php
index 6db2c32..67fab1c 100644
--- a/src/Plugin/diff/Layout/UnifiedFieldsDiffLayout.php
+++ b/src/Plugin/diff/Layout/UnifiedFieldsDiffLayout.php
@@ -108,7 +108,7 @@ class UnifiedFieldsDiffLayout extends DiffLayoutBase {
     $build['controls']['filter'] = [
       '#type' => 'item',
       '#title' => $this->t('Filter'),
-      '#wrapper_attributes' => ['class' => 'diff-controls__item'],
+      '#wrapper_attributes' => ['class' => ['diff-controls__item']],
       'options' => $this->buildFilterNavigation($entity, $left_revision, $right_revision, 'unified_fields', $active_filter),
     ];
 
diff --git a/src/Plugin/diff/Layout/VisualInlineDiffLayout.php b/src/Plugin/diff/Layout/VisualInlineDiffLayout.php
index 124cc8b..2cd888f 100644
--- a/src/Plugin/diff/Layout/VisualInlineDiffLayout.php
+++ b/src/Plugin/diff/Layout/VisualInlineDiffLayout.php
@@ -163,7 +163,7 @@ class VisualInlineDiffLayout extends DiffLayoutBase {
     $build['controls']['view_mode'] = [
       '#type' => 'item',
       '#title' => $this->t('View mode'),
-      '#wrapper_attributes' => ['class' => 'diff-controls__item'],
+      '#wrapper_attributes' => ['class' => ['diff-controls__item']],
       'filter' => [
         '#type' => 'operations',
         '#links' => $options,