That's part of the pdfmake library that Buttons uses for PDF creation. My guess is that you are using an old browser that doesn't support that (IE8,9 for example).
pdfmake doesn't degrade very well in older browsers. I sent a pull request with a fix, and it was accepted, but it hasn't been included in a release package of pdfmake yet.
Answers
That's part of the pdfmake library that Buttons uses for PDF creation. My guess is that you are using an old browser that doesn't support that (IE8,9 for example).
pdfmake doesn't degrade very well in older browsers. I sent a pull request with a fix, and it was accepted, but it hasn't been included in a release package of pdfmake yet.
Allan