It is a bug. we hope that Adobe will solve this problem.
Meanwhile you should do it in this way:
1) clic the plus button next to the empty field and enter the formula "partial paragraph" from the contextual menu
2) click between the two quotes (without typing anything)
3) place the mouse cursor on the OK button (just place without clic).
4) type the colon and quickly (before the error message appears) click the OK button
I found a solution on another site (I forget the source, sorry)—when I enter a delimiter, the quotes around the delimiter change from straight to curly (it's incredibly hard to see in the editing window, but it's happening), and the syntax error appears. If I enter my delimiter, then replace the curly quotes with straight quotes (on Mac, use shift+control+"), there is no syntax error any longer.