Turns out I had 2 methods named "setHeadings", and 2 methods named "getHeadings" with different signatures, and Stripes was getting confused and calling the wrong one.
So stripes was trying to call my "getHeadings" for the property, when it wanted a "getHeadings" for a resolution, so it could return a response.
The solution was to simply change my post methods name to "saveHeadings" instead of "setHeadings" to avoid the collision!