diff --git a/index.js b/index.js
index 003194a..0a8a394 100644
--- a/index.js
+++ b/index.js
@@ -9,6 +9,7 @@ const {Stringifier} = require('csv-stringify')
 const formatters = require('./lib')
 const getDependencies = require('./lib/deps')
 const pkg = require('./package.json')
+const {DEFAULT_AGENCY_ID} = require('./lib/agency')
 
 const convertGtfsToSql = async function* (files, opt = {}) {
 	opt = {
@@ -24,6 +25,8 @@ const convertGtfsToSql = async function* (files, opt = {}) {
 		statsByAgencyIdAndRouteIdAndStopAndHour: 'none',
 		statsActiveTripsByHour: 'none',
 		schema: 'public',
+		// todo: find something more helpful than falling back to Etc/GMT!
+		defaultTimezone: new Intl.DateTimeFormat().resolvedOptions().timeZone || 'Etc/GMT',
 		postgraphile: false,
 		postgraphilePassword: process.env.POSTGRAPHILE_PGPASSWORD || null,
 		postgrest: false,
@@ -208,6 +211,7 @@ LANGUAGE sql;
 	const nrOfRowsByName = new Map()
 	const workingState = {
 		nrOfRowsByName,
+		insertDefaultAgency: false,
 		onlyAgencyId: null,
 	}
 
@@ -217,6 +221,7 @@ LANGUAGE sql;
 	// However, because we have to use left join instead of an inner join in tables referencing `agency`, this prevents the PostgreSQL query planner from doing some filter pushdowns, e.g.
 	// - when querying `arrivals_departures` by route, stop, date and t_departure/t_arrival
 	{
+		let agencies = 0
 		for await (const agency of await readCsv('agency')) {
 			workingState.onlyAgencyId = agency.agency_id
 			if (++agencies >= 2) {
@@ -224,6 +229,11 @@ LANGUAGE sql;
 				break
 			}
 		}
+		// We insert a mock agency in order to use an inner join in tables referencing `agency`.
+		if (agencies === 0) {
+			workingState.insertDefaultAgency = true
+			workingState.onlyAgencyId = DEFAULT_AGENCY_ID
+		}
 	}
 
 	for (const name of order) {
diff --git a/lib/agency.js b/lib/agency.js
index f9e3697..7cd2b03 100644
--- a/lib/agency.js
+++ b/lib/agency.js
@@ -1,5 +1,7 @@
 'use strict'
 
+const DEFAULT_AGENCY_ID = 'default-agency'
+
 // https://gtfs.org/schedule/reference/#agencytxt
 const beforeAll = (opt) => `\
 CREATE TABLE "${opt.schema}".agency (
@@ -39,11 +41,32 @@ const formatAgencyRow = (a) => {
 	]
 }
 
-const afterAll = `\
+const afterAll = (opt, workingState) => {
+	let sql = `\
 \\.
 `
 
+	if (workingState.insertDefaultAgency) {
+		sql += `\
+INSERT INTO "${opt.schema}".agency (
+	agency_id,
+	agency_name,
+	agency_url,
+	agency_timezone
+) VALUES (
+	'${DEFAULT_AGENCY_ID}',
+	'implicit default agency, the CSV file doesn\\'t contain one',
+	'http://example.org',
+	'${opt.defaultTimezone}'
+);
+`
+	}
+
+	return sql
+}
+
 module.exports = {
+	DEFAULT_AGENCY_ID,
 	beforeAll,
 	formatRow: formatAgencyRow,
 	afterAll,
diff --git a/lib/stop_times.js b/lib/stop_times.js
index dd1c37c..a2ad1c9 100644
--- a/lib/stop_times.js
+++ b/lib/stop_times.js
@@ -225,14 +225,7 @@ WITH stop_times_based AS NOT MATERIALIZED (
 		LEFT JOIN "${opt.schema}".stops stations ON stops.parent_station = stations.stop_id
 		JOIN "${opt.schema}".trips ON s.trip_id = trips.trip_id
 		JOIN "${opt.schema}".routes ON trips.route_id = routes.route_id
-		LEFT JOIN "${opt.schema}".agency ON (
-			-- The GTFS spec allows routes.agency_id to be NULL if there is exactly one agency in the feed.
-			-- Note: We implicitly rely on other parts of the code base to validate that agency has just one row!
-			-- It seems that GTFS has allowed this at least since 2016:
-			-- https://github.com/google/transit/blame/217e9bf/gtfs/spec/en/reference.md#L544-L554
-			routes.agency_id IS NULL -- match first (and only) agency
-			OR routes.agency_id = agency.agency_id -- match by ID
-		)
+		JOIN "${opt.schema}".agency ON routes.agency_id = agency.agency_id
 		JOIN "${opt.schema}".service_days ON trips.service_id = service_days.service_id
 	)
 	-- todo: this slows down slightly
@@ -465,14 +458,7 @@ WITH stop_times_based AS NOT MATERIALIZED (
 			) AS to_wheelchair_boarding
 		FROM "${opt.schema}".trips
 		LEFT JOIN "${opt.schema}".routes ON trips.route_id = routes.route_id
-		LEFT JOIN "${opt.schema}".agency ON (
-			-- The GTFS spec allows routes.agency_id to be NULL if there is exactly one agency in the feed.
-			-- Note: We implicitly rely on other parts of the code base to validate that agency has just one row!
-			-- It seems that GTFS has allowed this at least since 2016:
-			-- https://github.com/google/transit/blame/217e9bf/gtfs/spec/en/reference.md#L544-L554
-			routes.agency_id IS NULL -- match first (and only) agency
-			OR routes.agency_id = agency.agency_id -- match by ID
-		)
+		JOIN "${opt.schema}".agency ON routes.agency_id = agency.agency_id
 		LEFT JOIN "${opt.schema}".stop_times ON trips.trip_id = stop_times.trip_id
 		LEFT JOIN "${opt.schema}".stops from_stops ON stop_times.stop_id = from_stops.stop_id
 		LEFT JOIN "${opt.schema}".stops from_stations ON from_stops.parent_station = from_stations.stop_id