--- a/testing/marionette/addon.js
+++ b/testing/marionette/addon.js
@@ -22,17 +22,17 @@ const ERRORS = {
   [-4]: "ERROR_FILE_ACCESS: There was an error accessing the filesystem.",
   [-5]: "ERROR_SIGNEDSTATE_REQUIRED: The addon must be signed and isn't.",
 };
 
 async function installAddon(file) {
   let install = await AddonManager.getInstallForFile(file);
 
   return new Promise((resolve, reject) => {
-    if (install.error != 0) {
+    if (install.error) {
       reject(new UnknownError(ERRORS[install.error]));
     }
 
     let addonId = install.addon.id;
 
     let success = install => {
       if (install.addon.id === addonId) {
         install.removeListener(listener);