Dedicated TTL flashes will give false readings and errors if no film is in the camera. This is because the light meter reading is taken from light that bounces off the film surface at the moment of exposure. As film is usually a dull mid-grey or brown colour, the light meter is calibrated for that shade. When there is no film in the camera, the light meter sees a black shiny surface and gets itself totally confused.
I came across this 'problem' over 20 years ago, testing a Pentax Super A with a dedicated flash without any film. I took it back to the dealer only to be sent away feeling very silly.
The light meter thats used for daylight (non-flash) photography is different and is not affected by dry running the camera.