Memory access errors

I have a vb6 dll which Im referencing in vb.net.

All seems fine until I close the form and then I get the message

'the instruction at 0x61a91157 referenced memory at
0x00000000. The memory cannot be read.'

The dll is using arrays of a structure, to hold file positions and a sortcode. The dll returns a tab delimited string indicating whether the sortcode is valid or not. I have 2 text boxes(to input sortcode and account number) on the form and a button to validate the details entered.

Does anyone have any ideas at all???

Thanks for your help in advance
Sue

Did you trying setting the reference to Nothing after you're done ?

Yeh I did that as well. I think its something to do with the garbage collection not freeing up the memory straight away.
I ended up upgrading the DLL into .net and amending a few things and its working fine.
Thanks for your help anyway.
Sue