iframe creates an w3c validation error (no replies)

Hello,

I'm using the iframe for opt-out and the w3c validator gives me this output:

Error Line 188, Column 118: & did not start a character reference. (& probably should have been escaped as &.)
…ce.de/piwik/index.php?module=CoreAdminHome&action=optOut&language=de"></iframe>

Any idea how I can avoid it? Is it a false positive or something I or the piwik dev team can change?

Thxs and kind regards,
BC