VBscript messagbox and validation

I'm working on a validation script for my ASP page. I'm validation with the isLength value and i want to see wich form field has been left zero. I can't put the form field in a variable My script has the folowing lines:

function isLength(val)
is len(val.value)>0 then
islength=true
else Msgbox "Value of field" & document.formname.formfiled& "can't bne zero"
val.select
isLength=false
end if

end fucntion

function check ()
if not islength(document.formname.formfield) then exit function
end if

msgbox ("Data is correct and can be submitted")
document.formname.submit()
end function

My problem is now the line where the msgbox is executed :

else Msgbox "Value of field" & document.formname.formfiled& "can't be zero"

i want in the messagebox the field which has been left zero and the message i get is

Value of field [object] can't be zero.

Can anyone help me thanx anyway