There are provided an image forming apparatus and a method of controlling the same are provided, which are capable of conveying an insert sheet of a proper page in carrying out a recovery process and simplifying the recovery process, and a storage medium storing a program for executing the method. When a printing start instruction is input and an insert mode has been set, an insert sheet is fed. It is determined whether the insert sheet is for a top page of a copy (bundle) or not. When the insert sheet for the top page is detected, a proper printing operation in the insert mode is started. If the insert sheet is not determined to be for the top page, it is judged that the insert sheet has not been correctly set on the inserter tray, and then the insert sheet is discharged to a non-sort tray. Then, insert sheets are discharged to the non-sort tray until the insert sheet for the top page is detected.