postgis - Operation on mixed SRID geometries

[I'm new to the site and my English is poor, so it is difficult for me to explain my problem. But there is no other person I can ask.]

My question is related to this exercise http://postgis.net/workshops/postgis-intro/joins.html, but it gives an error when I run it.

SELECT
subways.name AS subway_name,
neighborhoods.name AS neighborhood_name,
neighborhoods.boroname AS borough
FROM nyc_neighborhoods AS neighborhoods
JOIN nyc_subway_stations AS subways
ON ST_Contains(neighborhoods.geom, subways.geom)
WHERE subways.name = 'Broad St';

The error is Operation on mixed SRID geometries