POSTGIS Join tables on ST_ClosestPoint

In an attempt to join two rasters with a very small offset, the join on ST_ClosestPoint geometry column is logical to me:

SELECT s.geom, s.val, c.val
FROM s
JOIN c ON ST_ClosestPoint(s.geom , c.geom) = s.geom

But this does not return any results.

(PostGIS v2.2.1 and PostgreSQL v9.5)