[BUG?] Too early evaluation of "defined"

Why this code doesn't work?

Code:

macro call_cosh argument
{
local a
; Detect constant
virtual
  db `argument
  load a byte from $$
end virtual

  display '^^', a, '^^', 13, 10

  if a >= '0' & a <= '9' ; It is a constant
    push dword argument ; This needs to be different for better precision

    if argument eqtype 1.0
      fld dword [esp]
    else
      fild dword [esp]
    end if

    add esp, 4
  else ; It is a variable (needs more work as this is not necessarily true)
    display 13, 10, "__VARIABLE__", 13, 10
    if defined argument#.float ; Why is it evaluated so early?
      fld dword [esp]
    else
      fild dword [esp]
    end if
  end if

  call cosh ; Argument passed in ST(0)
}

struc dd [arg]
{
common
  .float:
  dd arg
}

call_cosh 1.0
call_cosh 1
call_cosh alpha
call_cosh beta

alpha dd 1
beta dd 1.0

cosh: ; Not implemented

This code works and shows that "1.0" nor "1" hits the ELSE body:

Code:

macro call_cosh argument
{
local a
; Detect constant
virtual
  db `argument
  load a byte from $$
end virtual

  display '^^', a, '^^', 13, 10

  if a >= '0' & a <= '9' ; It is a constant
    push dword argument ; This needs to be different for better precision

    if argument eqtype 1.0
      fld dword [esp]
    else
      fild dword [esp]
    end if

    add esp, 4
  else ; It is a variable (needs more work as this is not necessarily true)
    display "__VARIABLE__", 13, 10
    if 1
      fld dword [esp]
    else
      fild dword [esp]
    end if
  end if

  call cosh ; Argument passed in ST(0)
}


struc dd [arg]
{
common
  .float:
  dd arg
}

call_cosh 1.0
call_cosh 1
call_cosh alpha
call_cosh beta

alpha dd 1
beta dd 1.0

cosh: ; Not implemented

Output wrote:

^^1^^
^^1^^
^^a^^
__VARIABLE__
^^b^^
__VARIABLE__

[edit]It is not "defined" the problem actually, it is the ELSE block itself that gets "speculatively" evaluated when not needed[/edit]