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I'm sorry but that is not correct for the AE1, AE! Program or the Canon A1. I have owned and still use an AE! 50mm f1.8 and A1 50mm f1.8 since I bought them new in the 1970's and I can state that lenses can be changed whether the shutter is cocked or not. It is possible that the diaphragm has been set manually off the camera and needs to be reset, or there may be some damage to the pins/lever protruding from the back of the lens. I would suggest that the OP downloads the manual and reads it in conjuction with camera and lens in hand to understand the operation of the lens and possibly identify the problem.