Solution: either redefine your A_CANT method's arguments to t_atom_long, or define your type as A_DIRECT, and make use of the object_method_direct() macro, passing in a function prototype to the macro (also see under floating point how this is required for anything which previously was A_CANT with floating point values). Technically many of these will still work properly due to the nature of how integers are passed on the stack under x64, without any change, it is still best practice.