I might be wrong but I think you're using an older version of Decker. Possibly 1.54 or earlier.
The reason I think that is because Decker 1.55 introduced some changes to Lil. The description box under the preview is showing a "0" which suggests that your version of Decker is from before "nil" was introduced.
Could you update to a more recent version and let us know if that fixed it?