Accessing pointer to a polymorphic value with C_PTR

I have a Fortran function which compiled with previous versions of Intel Fortran Compiler (ifort), but is rejected by more recent versions (2021.6.0), and I hope to fix it. This function accepts a class(*), and passes its address and length to a C function:

!> Send an arbitrary scalar object.
function SendObject(my,obj) result(ok)
use, intrinsic :: iso_c_binding
implicit none
    class(Socket) :: my
    class(*), target, intent(in)  :: obj

    type(c_ptr)       :: ptr
    integer(c_size_t) :: len

    ptr = c_loc(obj)
    len = c_sizeof(obj)
    ok = my%SendData(ptr,len)

end function

ifort now emits an error:

error #9023: The argument to C_LOC must not be polymorphic.   [OBJ]
ptr = c_loc(obj)

While this function is polymorphic, in practice it is used only for a few simple structs containing scalars.

What is the best way to get this function "working" again, as well as it worked with previous compilers? I just need to get a C pointer to obj. Thanks for any help!