Freemarker include directive not working

We are running 2.2 and I am now having problems with the freemarker include directive. After many many tries and incarnations of trying to include a file, I've resorted to the most simple form I know. Here is a snippet of the FTL file:

<#assign FTLfilename = "navglobal01.ftl"/>
<#include "${FTLfilename}">

However, after editing the desired asset I get the following error:

Please correct the errors below then click Finish.

• error regenerating rendition using freemarker-include-file.ftl: freemarker.template.TemplateException: Error reading included file navglobal01.ftl

navglobal01.ftl resides in the same directory as the file including it, I'd prefer it to be in an /includes folder of it's own, but I am making this as simple as possible to begin with and I still can't get it to work.