I'm not sure what popup you are talking about and "everytime what?" Your formula looks good to me. Can you upload an example where you are having a problem? (Go Advanced>Manage Attachments)
Ahhh, I see. It's because you have 1 in single quotes. Is your tab called sheet 1 or sheet1? If it's the second, then you don't need the quotes. If it's the first, it should be around the entire thing 'sheet 1'!A2:A1000
Does that fix your problem?
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