[fasmg] 'invalid instruction' problem

To learn fasmg, I try to re-create x86 architecture

Code:

macro x86.parse_operand ns*, operand*
  match =byte? value, operand
    ns.size = X86_OPERAND_SIZE_BYTE
    x86.get_operand_type ns, value
  else match =word? value, operand
    ns.size = X86_OPERAND_SIZE_WORD
    x86.get_operand_type ns, value
  else match =dword? value, operand
    ns.size = X86_OPERAND_SIZE_DWORD
    x86.get_operand_type ns, value
  else
    ns.size = X86_OPERAND_SIZE_UNDEFINED
    x86.get_operand_type ns, operand
  end match
end macro

macro x86.get_operand_type ns*, value*
  match [addr], value
    ns.type = 'mem'
    ns.value = +addr
  else match segment:offset, value
    ns.type = 'farptr'
    ns.segment = +segment
    ns.value = +offset
    match [addr], offset
      ns.type = 'mem'
      ns.value = +addr
    end match
  else
    ns.type = 'imm'
    ns.value = +value        ; <<<<<<<<<<<<<<<<<<<
    if ns.value eq ns.value element 1
      if (ns.value metadata 1) metadata 1 relativeto x86.reg
        ns.type = 'reg'
      else if ns.value metadata 1 relativeto x86.sreg
        ns.type = 'sreg'
      end if
    end if
  end match
end macro

macro add? dest*, src*
  ;x86.parse_operand _operand1_, dest
  x86.parse_operand _operand2_, src

end macro

Marked string throws "Error: illegal instruction" for immediate operand:

Code:

add bx, 1

. It's not allowed to use variables with the same name (If I change "ns.value" to "ns.imm" orr anything else in the final ELSE clause, then code compiles without errors). But why? 'value' variables, as I see them, are in different namespaces. Could anyone clarify this behavior please?