No auto-completion with Clang Code Model on C++ project

When I enable the Clang code model on my C++ project, auto-complete stops working. I've tried restarting QtC after enabling the clang code modeling, but auto-completion still does not work (the menu of possible completions never appears), even if I press Ctrl-Space. The syntax highlighting works, and following symbols with F2 works at least as well as with the built-in model, but even if I do a simple auto-completion task like:

int abcdef;
abcd

in main() of my normal project, I get nothing.

I am referencing http://doc-snapshot.qt-project.org/qtcreator-3.3/creator-clang-codemodel.html for instructions on using the clang model.

Auto-completion seems to work pretty well in a dummy widgets project. If I type "ui." it doesn't automatically change it to "ui->", but if I type "ui->" it begins suggesting methods of ui.