Line tab won't load with "step" zoom changes

[RossThorn]

this error appeared when a line-width property used a "step" zoom transition and the "Line" tab in chartographer won't load:

here's the code that was tripping it up:

"line-width": [
        "step",
        ["zoom"],
        0.75,
        16, 1.5
      ],

as soon as I changed this to a single value or used "interpolate", it worked.

However, a step transition should also be able to be handled by chartographer.