struc ABC_BAR {.value rd 1}

virtual at 0
   ABC_BAR ABC_BAR
end virtual

macro foo var1*,prefix*,structure,var2 {
  mov eax,var1
  if ~ structure eq
     mov edx,[eax+prefix#_#structure#.#var2]
        ;the above line has an "invalid address" error
        ;if "structure" is empty. The line looks like this:
        ;"mov edx,[eax+ABC_#.]" after macro expansion
  end if
}

foo baz,ABC,BAR,value ;this line is okay
foo baz,ABC           ;this line fails with "invalid address"    

struc ABC_BAR {.value rd 1}

virtual at 0
   ABC_BAR ABC_BAR
end virtual

macro foo var1*,prefix*,structure,var2 {
  mov eax,var1
  if ~ structure eq xxx ;note the ugly "xxx"
     mov edx,[eax+prefix#_#structure#.#var2]
  end if
}

foo baz,ABC,BAR,value
foo baz,ABC,xxx    

  match any,structure
  \{
     mov edx,[eax+prefix#_#structure#.#var2]
  \}    

But you can use help of the "match" to save parser from handling such line.

BTW, do you think I should remove that "multiple #" feature? It is not even documented anymore.