We observed this behavior at "Planio":https://plan.io when invalid markdown syntax was input by the user, which caused an exception in the @link@ callback declared in source:trunk/lib/redmine/wiki_formatting/markdown/formatter.rb because @content@ was @nil@.

This exception seems to make the formatter instance unusable - the next time the formatter was used it crashed the whole ruby process. This seems to be related to "this redcarpet issue":https://github.com/vmg/redcarpet/issues/318.

The patch contains a test case and the (trivial) fix to take care of @content==nil@ in @#link@.