'FATAL: database "null" does not exist ' when accessing through a datasource

I've set up a datasource to access a database on the local machine. The
database was installed in the default location on the default port. Whenever
I try to make a connection to the database I get an error, usually this one.

org.postgresql.util.PSQLException: FATAL: database "null" does not exist

I've tried every conceivable permuation of the url (and a few inconceivable
ones). The datasource is set up like this:

DataSourceClass: org.postgresql.ds.PGSimpleDataSource
JNDI Name: jdbc/PGSimple
URL: jdbc:postgresql://localhost:5432/PGSQL1

And the code to access it is like this:

InitialContext ic = new InitialContext();
PGSimpleDataSource PGDS =
(PGSimpleDataSource) ic.lookup("jdbc/PGSimple);
Connection PGSimpleConn = PGDS.getConnection();
/* do Stuff /*
PGSimpleConn.close();

That code fails with the above error. But if I add this statement, it works
okay:

PGDS.setDatabaseName("PGSQL1");

I can't understand why I can't specify the databasename in the URL.

The container is OC4J running in Oracle Jdeveloper.

Any thoughts would be very much appreciated.

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