Marketing Cloud Barcodes

I’ve got the following code generating a barcode:

%%[ set @BarcodeURL = BarCodeURL(@Barcode, "Code128Auto", 100, 50)]%%

<p><img src="%%=v(@BarcodeURL)=%%"></p>

This works great apart from when it scan. When scanned the barcode says it has an invalid number. However, if the same barcode is manually entered it displays the amount and a valid voucher.

Is there something I’m missing in my code that would solve this issue? I noticed on the ampscript guide that I should be including a Checksum value for Code128 type barcodes but I’m not sure what this means/how to generate.

Thanks