Pandoc error 83

In the yaml header of my rmd file the line below generates the following error: pandoc-citeproc: Error parsing XML file apl.csl: apl.csl: openBinaryFile: does not exist (No such file or directory)

csl: apl.csl

However, when I change the line to read as below, no error is generated:

pandoc_args: [
'--csl', 'apl.csl'

I am rendering to both html and word and the apl.csl file is located in my working directory.