diff --git a/src/wp-admin/includes/schema.php b/src/wp-admin/includes/schema.php
index 8ea0b3680291c..099a35361c639 100644
--- a/src/wp-admin/includes/schema.php
+++ b/src/wp-admin/includes/schema.php
@@ -52,6 +52,13 @@ function wp_get_db_schema( $scope = 'all', $blog_id = null ) {
 	 */
 	$max_index_length = 191;
 
+	/*
+	 * comment_post_ID_parent_content indexes comment_content after two bigint columns.
+	 * Limit the content prefix to floor( ( 767 - 16 - 2 ) / 4 ) = 187 characters,
+	 * keeping the composite key below the 767-byte limit.
+	 */
+	$max_comment_content_index_length = 187;
+
 	// Blog-specific tables.
 	$blog_tables = "CREATE TABLE $wpdb->termmeta (
 	meta_id bigint(20) unsigned NOT NULL auto_increment,
@@ -116,6 +123,7 @@ function wp_get_db_schema( $scope = 'all', $blog_id = null ) {
 	user_id bigint(20) unsigned NOT NULL default '0',
 	PRIMARY KEY  (comment_ID),
 	KEY comment_post_ID (comment_post_ID),
+	KEY comment_post_ID_parent_content (comment_post_ID,comment_parent,comment_content($max_comment_content_index_length)),
 	KEY comment_approved_date_gmt (comment_approved,comment_date_gmt),
 	KEY comment_date_gmt (comment_date_gmt),
 	KEY comment_parent (comment_parent),
diff --git a/src/wp-includes/version.php b/src/wp-includes/version.php
index 934e5d0bb5369..364e97f70ce2a 100644
--- a/src/wp-includes/version.php
+++ b/src/wp-includes/version.php
@@ -23,7 +23,7 @@
  *
  * @global int $wp_db_version
  */
-$wp_db_version = 61833;
+$wp_db_version = 61834;
 
 /**
  * Holds the TinyMCE version.
diff --git a/tests/phpunit/tests/comment/wpAllowComment.php b/tests/phpunit/tests/comment/wpAllowComment.php
index 8b3de23fc194c..c3aa08c2bd605 100644
--- a/tests/phpunit/tests/comment/wpAllowComment.php
+++ b/tests/phpunit/tests/comment/wpAllowComment.php
@@ -71,4 +71,41 @@ public function test_die_as_duplicate_if_comment_author_name_and_emails_match()
 
 		$result = wp_allow_comment( $comment_data );
 	}
+
+	/**
+	 * @ticket 26858
+	 */
+	public function test_return_error_as_duplicate_if_comment_author_email_is_empty() {
+		$comment_id = self::factory()->comment->create(
+			array(
+				'comment_post_ID'      => self::$post_id,
+				'comment_approved'     => '1',
+				'comment_author'       => 'Anonymous Bob',
+				'comment_author_email' => '',
+				'comment_author_url'   => 'http://example.com',
+				'comment_content'      => 'Still a duplicate.',
+			)
+		);
+
+		$now          = time();
+		$comment_data = array(
+			'comment_post_ID'      => self::$post_id,
+			'comment_author'       => 'Anonymous Bob',
+			'comment_author_email' => '',
+			'comment_author_url'   => 'http://example.com',
+			'comment_content'      => 'Still a duplicate.',
+			'comment_author_IP'    => '192.168.0.1',
+			'comment_parent'       => 0,
+			'comment_date_gmt'     => gmdate( 'Y-m-d H:i:s', $now ),
+			'comment_agent'        => 'Bobbot/2.1',
+			'comment_type'         => '',
+		);
+
+		$result = wp_allow_comment( $comment_data, true );
+
+		wp_delete_comment( $comment_id, true );
+
+		$this->assertWPError( $result );
+		$this->assertSame( 'comment_duplicate', $result->get_error_code() );
+	}
 }