Possible error (or self confusion) in Discworld RPG

[LBEaston]

The rule on page 197 of the recent edition in the section called "Area" is confusing me.
It states "If a witch casts an area spell on a clearly defined location or community ... whose inhabitants mostly know or know of her, divide the effective radius by five. If the people don't know her, she may be able to fix that immediately before casting by performing..."

As I understand it, the witch's spell will be less powerful if people know of her, but then why would she want to "fix" that? Shouldn't it be "multiply" instead of "divide"? Witches are supposed to get (at least some of) their power from people believing that witches have power. Which would also suggest that more people knowing of them would make their spells more powerful.

Edit: I wanted to get a second opinion before submitting it as errata.