mumamo-fontify-region: Wrong number of arguments

[statico]

I've set up jekyll.el and jekyll-mumamo.el in my init.el:

;; Jekyll Blog Editing Mode
(require 'jekyll)
(add-to-list 'jekyll-modes-list '("xml" 'xml-mode))
(add-to-list 'jekyll-modes-list '("html" 'html-mode))
(add-to-list 'auto-mode-alist '("_\\(posts\\|drafts\\).*\\.md$" . jekyll-markdown-mumamo-mode))

Unfortunately, I get this when opening a simple Markdown post:

mumamo-fontify-region: Wrong number of arguments: (lambda (pos max) "Find yaml header.  Return range and 'yaml-mode.
See `mumamo-possible-chunk-forward' for POS and MAX." (mumamo-quick-chunk-forward pos max "---" "---" (quote borders) (quote yaml-mode))), 3

Any ideas?