Can't add cover in Calibre -- get "svg-error" (the property 'svg' should be declared)

When I add a cover to my ePub file, I get this Validator error:

"ERROR: OEBPS/titlepage.xhtml -- The property 'svg' should be declared in the OPF file."

Could anyone please tell me how I fix this?

EDIT: I've tried adding the cover in the two ways:

1) Using Calibre's "Add cover" option.
2) Adding the image as a New File, and then adding that file as a cover.

But both ways result in the same error. Am I missing a third way?