I'm currently having a problem retrieving a vintage matrix for certain mnemonic codes. I am able to use the get_epit_vintage_matrix() function given below for the mnemonics "USGDP...D" and "USM2....B" but not for "USIPTOT.G" or "USCNFCONQ". There seems to be a "KeyError" but I'm not sure why. I have attached screenshots below of the function and of the error message received. Can somebody please assist me with this issue? Thanks!