Qt and no moc_*.cpp file

I'm developing a simple Qt 4 app and making my own dialog. I subclassed QDialog, inserted the Q_OBJECT macro in the class declaration block, and... I get

[Linker error] undefined reference to `vtable for MyDialog' and there is no moc_MyDialog.cpp generated by the moc compiler.

I am using Qt 4.1.3 on Windows XP and mingw. I followed the build process from the Qt-supplied build shell. I used qmake to create make files and compiled everything with a make command.

I have other classes that subclass QPushButton and QObject respectively, but they compile OK. I can't find any differences between them and the broken one.

There must be missing something in the broken class, but I'm unable to spot it.