Index: test/all.js
===================================================================
--- a/test/all.js
+++ b/test/all.js
@@ -29,8 +29,8 @@
   if (spec)
   {
     if (spec == "installed")
-      return Promise.resolve("");
-    if (spec.startswith("path:"))
+      return Promise.resolve(null);
+    if (spec.startsWith("path:"))
       return Promise.resolve(spec.substr(5));
     if (spec.startsWith("download:"))
       version = spec.substr(9);
Index: test/browsers/chromium.js
===================================================================
--- a/test/browsers/chromium.js
+++ b/test/browsers/chromium.js
@@ -39,10 +39,12 @@
 exports.getDriver = function(browserBinary, devenvPath)
 {
   let options = new chrome.Options()
-    .setChromeBinaryPath(browserBinary)
     .addArguments("--no-sandbox")
     .addArguments(`load-extension=${devenvPath}`);
 
+  if (browserBinary != null)
+    options.setChromeBinaryPath(browserBinary);
+
   return new webdriver.Builder()
     .forBrowser("chrome")
     .setChromeOptions(options)
Index: test/browsers/firefox.js
===================================================================
--- a/test/browsers/firefox.js
+++ b/test/browsers/firefox.js
@@ -33,7 +33,10 @@
 
 exports.getDriver = function(browserBinary, devenvPath)
 {
-  let options = new firefox.Options().setBinary(browserBinary).headless();
+  let options = new firefox.Options().headless();
+  if (browserBinary != null)
+    options.setBinary(browserBinary);
+
   let driver = new webdriver.Builder()
     .forBrowser("firefox")
     .setFirefoxOptions(options)