Highlight n-th gantttitlelist element in bold

I’m trying to make a pgfgantt chart and highlight the gantttitlelist elements in regular intervals by making them bold.

To illustrate, here is an example of what I tried:

documentclass{article}

usepackage{pgfgantt}

begin{document}

begin{ganttchart}{1}{36}
  gantttitle{Months}{36} 
  gantttitlelist{1,...,36}{1}
  gantttitlelist[title/.style={font=bfseries}]{12}{1}
  gantttitlelist[title/.style={font=bfseries}]{24}{1}
  gantttitlelist[title/.style={font=bfseries}]{36}{1}
end{ganttchart}

end{document}

The resulting chart has no bold elements, but I would like that every 12th element, so 12, 24 and 36 are displayed using a bold font.

Any ideas of what I’m missing here?