ORDER BY with UNION

Having trouble with the following union query...

SELECT
table_catalog AS "databaseName",
table_schema AS "schemaName",
table_name AS "tableName",
'' AS "primaryKeyName",
column_name AS "columnMappings"
FROM information_schema.columns
WHERE table_schema NOT IN ('information_schema','pg_catalog')
UNION ALL
SELECT
table_catalog AS "databaseName",
table_schema AS "schemaName",
table_name AS "tableName",
constraint_name AS "primaryKeyName",
column_name AS "columnMappings"
FROM information_schema.key_column_usage
WHERE constraint_name LIKE 'pk_%'
-- ORDER BY
-- table_catalog,
-- table_schema,
-- table_name,
-- constraint_name,
-- ordinal_position,
-- column_name
;

This works fine as above but as soon as I reintroduce the ORDER BY clause I get the syntax error...
ERROR: column "table_catalog" does not exist
LINE 19: table_catalog,
^
********** Error **********
ERROR: column "table_catalog" does not exist
SQL state: 42703
Character: 667

From the documentation I infer that ORDER BY should work with UNION, so where am I going wrong?
Any help please...