diff --git a/src/config.coffee b/src/config.coffee
index 722ad663..e7fdddf4 100644
--- a/src/config.coffee
+++ b/src/config.coffee
@@ -1,3 +1,12 @@
+checkInt = (s) ->
+	# Make sure `s` exists and is not an empty string.
+	if !s
+		return
+	i = parseInt(s, 10)
+	if isNaN(i)
+		return
+	return i
+
 module.exports = config =
 	apiEndpoint: process.env.API_ENDPOINT ? 'https://api.resin.io'
 	registryEndpoint: process.env.REGISTRY_ENDPOINT ? 'registry.resin.io'
@@ -9,8 +18,8 @@ module.exports = config =
 	localImage: process.env.SUPERVISOR_IMAGE ? 'resin/rpi-supervisor'
 	configMountPoint: process.env.CONFIG_MOUNT_POINT ? '/mnt/mmcblk0p1/config.json'
 	ledFile: process.env.LED_FILE ? '/sys/class/leds/led0/brightness'
-	bootstrapRetryDelay: process.env.BOOTSTRAP_RETRY_DELAY_MS ? 30000
-	restartSuccessTimeout: process.env.RESTART_SUCCESS_TIMEOUT ? 60000
+	bootstrapRetryDelay: checkInt(process.env.BOOTSTRAP_RETRY_DELAY_MS) ? 30000
+	restartSuccessTimeout: checkInt(process.env.RESTART_SUCCESS_TIMEOUT) ? 60000
 	successMessage: 'SUPERVISOR OK'
 
 config.heartbeatEndpoint = config.apiEndpoint + '/ping'