pgstattuple_approx for toasted table

Hi,

I was testing pgstattuple and I realized that pgstattuple is working on toasted table but pgstattuple_approx is raising the next error msg : 

ERROR:  "pg_toast_18292" is not a table or materialized view

ahm, is that because the pgstattuple_approx uses visibility map ? Can someone explain ? tnx.