Selenium works locally but not from dev Google App Engine

I have simple Selenium code that opens a page, fills out a form, and submits it. In a simple project that has only a main(String[] args) method, it works fine. When that same code runs on the Google App Engine devserver, it fails. webElement.sendKeys() seems to have no effect. (The web element, which is a text input开发者_运维知识库, remains empty.)

Curiously, the following compiles when it's in the GAE project:

usernameInput.sendKeys("foo");

But when I move it to the simple project, I have to change it to:

usernameInput.sendKeys(new CharSequence[] {"foo"});

Why might that be? Could the issues be related?

Update: This fails for the default Google login, but it works for the Facebook login. I'm not sure why this would be.

When I upload to production, it seems to fail silently.