diff --git a/script/tool/lib/src/firebase_test_lab_command.dart b/script/tool/lib/src/firebase_test_lab_command.dart
index ff7d05bf4e..d4b8382f3b 100644
--- a/script/tool/lib/src/firebase_test_lab_command.dart
+++ b/script/tool/lib/src/firebase_test_lab_command.dart
@@ -27,8 +27,8 @@ class FirebaseTestLabCommand extends PluginCommand {
       help: 'The Firebase project name.',
     );
     argParser.addOption('service-key',
-        defaultsTo:
-            p.join(io.Platform.environment['HOME'], 'gcloud-service-key.json'));
+        defaultsTo: p.join(
+            io.Platform.environment['HOME'] ?? '/', 'gcloud-service-key.json'));
     argParser.addOption('test-run-id',
         defaultsTo: Uuid().v4(),
         help: