postgres_fdw cannot connect to server on Amazon RDS

I have two Postgres 9.3.5 instances in RDS, both in one security group that allows all inbound traffic from within the security group and all outbound traffic. I'm trying to set up one database to be able to select from a few tables from the other via postgres_fdw.

I've created the server -

create server master 
foreign data wrapper postgres_fdw 
OPTIONS (dbname 'main', 
         host 'myinstance.xxxxx.amazonaws.com');

as well as the requisite user mapping and foreign table -

create foreign table condition_fdw (
    cond_id integer,
    cond_name text
) server master options(table_name 'condition', schema_name 'data');

However, a simple select count(*) from condition_fdw gives me

ERROR:  could not connect to server "master"
DETAIL:  could not connect to server: Connection timed out
        Is the server running on host "myinstance.xxxxxx.amazonaws.com" (xx.xx.xx.xx) and accepting
        TCP/IP connections on port 5432?

I can connect to both databases via psql from an EC2 instance. I know until recently RDS didn't support postgres_fdw, but I'm running the newer versions that do.

In the create server statement, I have tried replacing "myinstance.xxxxxx.amazonaws.com" with the IP address it resolves to, no luck.

Any ideas?