zlib compress/uncompress banks

Well, what I thought would be a simple job of writing a data packer is turning into a complete nightmare!

I am trying to compress/uncompress a bank. This code was posted a while ago:

Import Pub.ZLib

Function CompressBank:TBank( bank:TBank )
	Local size=bank.Size()
	Local out_size=size+size/10+32
	Local out:TBank=TBank.Create( out_size )
	compress out.Buf()+4,out_size,bank.Buf(),size
	out.PokeByte 0,size
	out.PokeByte 1,size Shr 8
	out.PokeByte 2,size Shr 16
	out.PokeByte 3,size Shr 24
	out.Resize out_size+4
	Return out
End Function

Function UncompressBank:TBank( bank:TBank )
	Local out_size
	out_size:|bank.PeekByte(0)
	out_size:|bank.PeekByte(1) Shl 8
	out_size:|bank.PeekByte(2) Shl 16
	out_size:|bank.PeekByte(3) Shl 24
	Local out:TBank=TBank.Create( out_size )
	uncompress out.Buf(),out_size,bank.Buf()+4,bank.Size()-4
	Return out
End Function

but it does not seem to compress, and the uncompress function errors. Has anyone actually got this (or something similar) working?