selector-extend() and selector-replace() fail to parse valid inputs for $extendee

[nex3]

The function selector-extend("c.d", (c, ".d"), ".e") should return c.d, .e, but instead it throws Internal Error: Invalid UTF-8.

[nex3]

It looks like this is even broader than just failing to parse non-string inputs. It also fails to parse pseudo selectors with punctuation-y arguments. For example, selector-extend("a", ":b(^)", "c") also throws Internal Error: Invalid UTF-8.

[nex3]

In fact, it looks like punctuation in selector arguments causes Internal Error: Invalid UTF-8 in any argument:

• selector-extend(":a(^)", "b", "c")
• selector-extend("a", "b", ":c(^)")