Why I am getting full code of user defined command in rmarkdown?

---
documentclass: report
classoption: twoside
fontsize: 12pt
output: 
  pdf_document: 
    latex_engine: xelatex
    keep_tex: true
header-includes: |
  usepackage{geometry}
  usepackage{tikz}
  newcommand{zero}{centerline{tikz{node[circle, fill=gray, minimum size=0.4linewidth] {};}}}
---
zero

I am wondering why in the tex file I am not getting

begin{document}
zero
end{document}

instead I get the full code

begin{document}
centerline{tikz{node[circle, fill=gray, minimum size=0.4linewidth] {};}}
end{document}