How to use allocatable character in a Fortran common block?

Question

I am trying to have an allocatable character inside a common block, in Fortran 77. But the compiler always complains

Error: COMMON attribute conflicts with ALLOCATABLE attribute at (1)

The code I had is

      subroutine word()
          character (len = :), allocatable :: a
          common /a/ a
          a = 'word'
      end subroutine word

      program main
          character (len = :), allocatable :: a
          common /a/ a
          call word()
          print *, a
      end program main

The only way that I saw to make it work is not to use allocatable

      subroutine word()
          character * 4 :: a
          common /a/ a
          a = 'word'
      end subroutine word

      program main
          character * 4 :: a
          common /a/ a
          call word()
          print *, a
      end program main

But this would be cumbersome because I would have to know the length of a character in advance.

How could I make allocatable to work in a common block?