diff --git a/lib/cli/cli.js b/lib/cli/cli.js
index a26acbb9c..833dfc939 100644
--- a/lib/cli/cli.js
+++ b/lib/cli/cli.js
@@ -675,6 +675,7 @@ module.exports.parseCommandLine = function parseCommandLine() {
     })
     .option('browsertime.firefox.android.deviceSerial', {
       alias: 'firefox.android.deviceSerial',
+      type: 'string',
       describe:
         'Choose which device to use. If you do not set it, first device will be used.',
       group: 'Firefox'
@@ -767,6 +768,7 @@ module.exports.parseCommandLine = function parseCommandLine() {
     })
     .option('browsertime.chrome.android.deviceSerial', {
       alias: 'chrome.android.deviceSerial',
+      type: 'string',
       describe:
         'Choose which device to use. If you do not set it, the first found device will be used.',
       group: 'Chrome'