Theme error - expected string in theme/partials/li.html

I run Hugo 0.15 and installed Hugo’s themes.

When I run hugo server --theme=cocoa --buildDrafts I get this error:

ERROR: 2016/01/01 template: theme/partials/li.html:2:43: executing “theme/partials/li.html” at <.Site.Params.DateFor…>: invalid value; expected string in theme/partials/li.html

Add this to the end of your config.toml:

          DateForm = "Mon, Jan 2, 2006"

Note: If you already have a params section, you must skip the header.

On an added note: We should try to get a more uniform parameter set up of all the themes, but until then, you can get some hints in this file:

These are the main parameters used to get all the demo sites on to work.

I got this error as well. I checked config.toml and this line is already included under params for me, but I still get the error. What else could be the issue?