A key feature of this macro is that it only requires you to select the actual text of the footnote. At first blush, I thought it was necessary to include the body of the text in which the footnotes occur BUT that is not the case. I too was getting a type 13 error but I figured out it was because I was misunderstanding how convenient the macro is.

In the example document:
If you select the following text (which you might do if you thought the macro required the text of the footnote to be selected along with the footnote):
"This is a sentence."1 This is a sentence. This is a sentence.2
This is a new paragraph. This is a sentence.3 This is a sentence.4 This is a sentence.
1 This is a footnote.
2 This is a footnote.
3 This is a footnote.
4 This is a footnote.

You will get a type 13 error every time.

That's because the macro actually uses the fact that the text of the footnote always begins with a number. But body text doesn't follow that convention.
So the correct selection is:
“This is a sentence.”1 This is a sentence. This is a sentence.2
This is a new paragraph. This is a sentence.3 This is a sentence.4 This is a sentence.
1 This is a footnote.
2 This is a footnote.
3 This is a footnote.
4 This is a footnote.

This is a well-written and simple macro.