Spurious debugging message: can't find linker symbol for virtual table

At a breakpoint in a QMainWindow sub-class, the debugger reports

can't find linker symbol for virtual table for `CodeItem' value
found `QMenuPrivate::popupGeometry(int) const' instead

when it enters a method, but there is no error. If you put a breakpoint at MainWindow::qt_static_metacall(), then you get a similar error:

can't find linker symbol for virtual table for `QWidget' value
found `QMenuPrivate::popupGeometry(int) const' instead

In neither case is there actually an error.