Output statement overflows record on writing a namelist to a string

This piece of code,

program dong
  type x
     integer :: a,b,c
  end type x

  type(x) :: xx

  character(len=65536) :: str
  namelist /zac/ xx

  write(str,nml=zac)
  print *, trim(str)
end program dong

breaks down with the following runtime error:

forrtl: severe (66): output statement overflows record, unit -5, file Internal NML Write

Why? 

Oh, and ifort is 16.0.2