diff --git a/core/includes/theme.inc b/core/includes/theme.inc
index f8febe712b..e01732ad3a 100644
--- a/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -815,13 +815,23 @@ function template_preprocess_image(&$variables) {
     $variables['attributes']['srcset'] = implode(', ', $srcset);
   }
 
+  // Ensure that attributes can be accessed as an array, for PHP 7.4
+  // compatibility.
+  if ($variables['attributes'] instanceof Attribute) {
+    $attributes_array = $variables['attributes']->toArray();
+  }
+  elseif (is_array($variables['attributes'])) {
+    $attributes_array = $variables['attributes'];
+  }
+  else {
+    $attributes_array = [];
+  }
+
   foreach (['width', 'height', 'alt', 'title', 'sizes'] as $key) {
     if (isset($variables[$key])) {
       // If the property has already been defined in the attributes, do not
       // override, including NULL.
-      // @todo the cast to array below is needed to get tests pass on PHP 7.4,
-      // but requires a separate issue to fix.
-      if (array_key_exists($key, (array) $variables['attributes'])) {
+      if (array_key_exists($key, $attributes_array)) {
         continue;
       }
       $variables['attributes'][$key] = $variables[$key];