diff -u b/core/includes/theme.inc b/core/includes/theme.inc
--- b/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -1828,7 +1828,7 @@
       $items['pages'][$i]['attributes'] = new Attribute();
       if ($i == $pager_current) {
         $variables['current'] = $i;
-        $items['pages'][$i]['attributes']['aria-current'] = t('Current page');
+        $items['pages'][$i]['attributes']->setAttribute('aria-current', t('Current page'));
       }
     }
     // Add an ellipsis if there are further next pages.
diff -u b/core/modules/system/tests/src/Unit/Pager/PreprocessPagerTest.php b/core/modules/system/tests/src/Unit/Pager/PreprocessPagerTest.php
--- b/core/modules/system/tests/src/Unit/Pager/PreprocessPagerTest.php
+++ b/core/modules/system/tests/src/Unit/Pager/PreprocessPagerTest.php
@@ -52,6 +52,27 @@
     require_once $this->root . '/core/includes/theme.inc';
     $variables = [
       'pager' => [
+        '#element' => '',
+        '#parameters' => [],
+        '#quantity' => '',
+        '#route_name' => '',
+        '#tags' => '',
+      ],
+    ];
+    template_preprocess_pager($variables);
+
+    $this->assertEquals(['first', 'previous'], array_keys($variables['items']));
+  }
+
+  /**
+   * Tests template_preprocess_pager() when a #quantity value is passed.
+   *
+   * @covers ::template_preprocess_pager
+   */
+  public function testQuantitySet() {
+    require_once $this->root . '/core/includes/theme.inc';
+    $variables = [
+      'pager' => [
         '#element' => '2',
         '#parameters' => [],
         '#quantity' => '2',