‘undefined’ in code…..why????

I am using this javascript to pic random images for my website. Every once in awhile I get a ‘undefined’ instead of a pic. Why is this happening?

[CODE]HEAD:
<SCRIPT LANGUAGE=”JavaScript”>
<!– Begin
var maxAdNo = 6
var adNo
var myAd = new Array()
myAd[0] = ‘<img src=”front1.jpg” border=”0″ height=350 width=480>’
myAd[1] = ‘<img src=”front2.jpg” border=”0″ height=350 width=480>’
myAd[2] = ‘<img src=”front3.jpg” border=”0″ height=350 width=480>’
myAd[3] = ‘<img src=”front4.jpg” border=”0″ height=350 width=480>’
myAd[4] = ‘<img src=”front5.jpg” border=”0″ height=350 width=480>’
myAd[5] = ‘<img src=”front6.jpg” border=”0″ height=350 width=480>’
// End –>
</script>

BODY:
<SCRIPT LANGUAGE=”JavaScript”>
<!– Begin
adNo = Math.round(Math.random() * maxAdNo)
document.write(myAd[adNo])
// End –>
</script>[/CODE]

Thanks for the help

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