error with getting properties of Businesspartner

I have an addon which contains the following line (where BPCard is a businesspartner object):

If (BPCard.Properties(20).ToString = "tNO") Then

This was working good, but since we've installed patch level 20 we are getting the following error:

<b>Property '' of 'BusinessPartner' is invalid</b>

The weird thing is when i put down 3 instead of 20, i get the error: <b>Property 'perties' of 'BusinessPartner' is invalid</b> and when i put down 1 i get <b>Property 'roperties' of 'BusinessPartner' is invalid</b> and when i put down 6 i get <b>Property 'ties' of 'BusinessPartner' is invalid</b>, etc