sql-drop failed on empty database

I was trying to drop tables from empty postgres database and got this error message:

drush sql-drop
Do you really want to drop all tables? (y/n): y
psql:/tmp/drush_cA0NAj:1: ERROR:  syntax error at end of input
LINE 1: DROP TABLE 
                   ^

Here the code from _drush_sql_drop():

if ($tables = drush_shell_exec_output()) {
    ...
}
else {
  drush_log(dt('No tables to drop.'), 'ok');
}

In my case if ($tables = drush_shell_exec_output()) evaluates to TRUE because query result contains a header.

postgres=> select tablename from pg_tables where schemaname='public';
 tablename 
-----------
(0 rows)