diff --git a/core/modules/image/src/Plugin/Filter/FilterImageStyle.php b/core/modules/image/src/Plugin/Filter/FilterImageStyle.php
index 5791530..7926f41 100644
--- a/core/modules/image/src/Plugin/Filter/FilterImageStyle.php
+++ b/core/modules/image/src/Plugin/Filter/FilterImageStyle.php
@@ -149,6 +149,7 @@ protected function loadImageStyles() {
    *   The UUID for the file.
    *
    * @return array
+   *   The image information.
    */
   protected function getImageInfo($file_uuid) {
     /**
@@ -178,7 +179,8 @@ protected function getImageInfo($file_uuid) {
    * @param \DOMNode $dom_node
    *   The DOM node for the img element.
    *
-   * @return \DOMNode
+   * @return array
+   *  The attributes array.
    */
   protected function prepareImageAttributes($dom_node) {
     // Make sure all non-regenerated attributes are retained.
@@ -193,7 +195,7 @@ protected function prepareImageAttributes($dom_node) {
       $attributes[$attr->name] = $attr->value;
     }
 
-    return $dom_node;
+    return $attributes;
   }
 
   /**
@@ -207,8 +209,9 @@ protected function prepareImageAttributes($dom_node) {
    *  The DOM node for the image element.
    *
    * @return string
+   *   The img element with the image style applied.
    */
-  protected function getImageStyleHtml($file_uuid, $image_style_id, $dom_node) {
+  protected function getImageStyleHtml($file_uuid, $image_style_id, \DOMNode $dom_node) {
     $image_info = $this->getImageInfo($file_uuid);
     $image_uri = $image_info['uri'];
     $image_width = $image_info['width'];
diff --git a/core/modules/image/tests/src/Unit/FilterImageStyleTest.php b/core/modules/image/tests/src/Unit/FilterImageStyleTest.php
index a5455cc..d1f9e4f 100644
--- a/core/modules/image/tests/src/Unit/FilterImageStyleTest.php
+++ b/core/modules/image/tests/src/Unit/FilterImageStyleTest.php
@@ -79,13 +79,20 @@ public function testProcessWithImage() {
     $original_alt = 'A wooly mammoth trumpets as a crevasse breaks open in the glacier.';
 
     $original_img = '<img src="' . $original_src .'" width="' . $original_width .'" height="' . $original_height .'" alt="' . $original_alt .'" data-entity-uuid="' . $original_uuid . '" data-entity-type="file" data-image-style="' . $original_image_style . '" />';
+    $original_img_doc = new \DOMDocument($original_img);
+    $original_img_node = $original_img_doc->createElement('img');
 
     $processed_img = '<img alt="' . $original_alt .'" data-entity-type="file" />';
+    $processed_img_doc = new \DOMDocument($processed_img);
+    $processed_img_node = $processed_img_doc->createElement('img');
 
     $generated_src = 'styles/medium/public/image.png';
     $generated_width = '200';
     $generated_height = '150';
+
     $generated_img = '<img src="' . $generated_src .'" width="' . $generated_width .'" height="' . $generated_height .'" alt="' . $original_alt .'" data-entity-type="file" />';
+    $generated_img_doc = new \DOMDocument($generated_img);
+    $generated_img_node = $generated_img_doc->createElement('img');
 
     $this->filterImageStyle
       ->method('loadImageStyles')
@@ -109,20 +116,20 @@ public function testProcessWithImage() {
     $this->filterImageStyle
       ->method('prepareImageAttributes')
       ->with(
-        $this->equalTo($original_img)
+        $this->equalTo($original_img_node)
       )
-      ->willReturn($processed_img);
+      ->willReturn($processed_img_node);
 
     $this->filterImageStyle
       ->method('getImageStyleHtml')
       ->with(
         $this->equalTo($original_uuid),
         $this->equalTo($original_image_style),
-        $this->equalTo($original_img)
+        $this->equalTo($original_img_node)
       )
-      ->willReturn($generated_img);
+      ->willReturn($generated_img_node);
 
     $output = $this->filterImageStyle->process($original_img, 'en');
-    $this->assertEquals($generated_img, $output);
+    $this->assertEquals($generated_img_node, $output);
   }
 }