I am trying to place the value of the variables into this url but I keep getting this error on my variables:

<script language = javascript>
var Address1 = FORM1.Address1.value
var Address2 = FORM1.Address2.value
var Province1 = FORM1.Province1.value
var Province2 = FORM1.Province2.value

window.open('http://www.mapquest.com/directions/main.adp?go=1&do=nw&ct=NA&1y=US&1a=" & Address1 & "&1p=&1c=" & City1 & " &1s=" & Province1 & "&1z=&1ah=&2y=US&2a=" & Address2 & "&2p=&2c=" & City2 & "&2s=" & Province2.value & "&2z=&2ah=&lr=2&x=66&y=11','NewWindow')
</script>

I keep getting the error:
Line 17: Error: 'FORM1.Address1.value' is null or not an object

My form is named FORM1...

What other ways can I try for example:

window.document.FORM1.Address1.value