diff -u b/core/lib/Drupal/Core/Template/Attribute.php b/core/lib/Drupal/Core/Template/Attribute.php
--- b/core/lib/Drupal/Core/Template/Attribute.php
+++ b/core/lib/Drupal/Core/Template/Attribute.php
@@ -337,10 +337,10 @@
     $count = 0;
     foreach ($this->storage as $value) {
       // Only count a value if it will be rendered, see twig_test_empty().
-      if ($value::RENDER_EMPTY_ATTRIBUTE) {
-        $count++;
+      if ($value instanceof \Traversable) {
+        $count += iterator_count($value);
       }
-      elseif (!$value::RENDER_EMPTY_ATTRIBUTE && !empty($value->value())) {
+      else {
         $count++;
       }
     }
diff -u b/core/tests/Drupal/Tests/Core/Template/AttributeTest.php b/core/tests/Drupal/Tests/Core/Template/AttributeTest.php
--- b/core/tests/Drupal/Tests/Core/Template/AttributeTest.php
+++ b/core/tests/Drupal/Tests/Core/Template/AttributeTest.php
@@ -532,9 +532,9 @@
     $attributes['class'][] = 'cat';
     $this->assertSame(1, count($attributes));
     $attributes['class'][] = 'tabby';
-    $this->assertSame(1, count($attributes));
-    $attributes['id'] = 'test_id';
     $this->assertSame(2, count($attributes));
+    $attributes['id'] = 'test_id';
+    $this->assertSame(3, count($attributes));
     $attributes['class'] = [];
     $this->assertSame(1, count($attributes));
     unset($attributes['id']);