Entity id is NULL, cannot insert into download_count db table

I was getting an error page when my site's users tried to download files from nodes. The watchdog table showed three errors:

PDOException: SQLSTATE23000: Integrity constraint violation: 1048 Column 'id' cannot be null: INSERT INTO {download_count} (fid, uid, type, id, ip_address, referrer, timestamp) VALUES (:db_insert_placeholder_0, :db_insert_placeholder_1, :db_insert_placeholder_2, :db_insert_placeholder_3, :db_insert_placeholder_4, :db_insert_placeholder_5, :db_insert_placeholder_6); Array ( [:db_insert_placeholder_0] => 52 [:db_insert_placeholder_1] => 32 [:db_insert_placeholder_2] => node [:db_insert_placeholder_3] => [:db_insert_placeholder_4] => 192.168.0.1 [:db_insert_placeholder_5] => http://example.com/list [:db_insert_placeholder_6] => 1473690039 ) in download_count_file_download_access_alter() (line 196 of .../download_count/download_count.module).

Notice: Array to string conversion in download_count_file_download_access_alter() (line 191 of .../download_count/download_count.module).

Notice: Undefined property: stdClass::$Array in download_count_file_download_access_alter() (line 191 of .../download_count/download_count.module)

The code causing the problem is in the download_count_file_download_access_alter():

  $id = db_insert('download_count')
  ->fields(array(
    'fid' => $file['fid'],
    'uid' => $user->uid,
    'type' => $entity_type,
    'id' => $entity->$entity_info['entity keys']['id'],
    'ip_address' => $ip,
    'referrer' => $referrer,
    'timestamp' => $time,
  ))
  ->execute();

Line 191 is 'id' => $entity->$entity_info['entity keys']['id'],.

Through debugging, I've found the id is NULL, although my $entity is a proper node object, and $entity_info['entity keys']['id'] returns 'nid'.

Adding curly brackets around the property name in line 191 as 'id' => $entity->{$entity_info['entity keys']['id']}, solves this problem.

I'll create a patch shortly.