When you use the stopped-down AE mode with the FD lens, do not push in the stop-down lever with the lens set on the "A" mark. If you do "E" marks indicating an error will appear to ware you of incorrect operation (EEEE EE in the viewfinder and EEE on the display panel) and the shutter will lock

In this case push in the stop-down lever again to clear the stopped-down AR or remove the lens from the "A" mark. The error display will then disappear.