What happens is that when makeCursor is called from subCursor, it will always have 4 arguments and https://github.com/facebook/immutable-js/blob/257b92c6887ae787da9c94fbabe34056014ffc93/contrib/cursor/index.js#L223 doesn't get run and thus size is undefined.

So either the check has to be if (typeof value === 'undefined') or the subCursor has to to do call magic.