Cannot Capitalize citeauthor using capitalisewords

I am dealing with a citation author whose surname is "dos Santos". I want to capitalize the "dos" since it appears in the beginning of a sentence. Here is my code for the main paper.

documentclass[conference]{IEEEtran}
%IEEEoverridecommandlockouts
% The preceding line is only needed to identify funding in the
%first footnote. If that is
%unneeded, please comment it out.
%usepackage{cite}
usepackage{amsmath,amssymb,amsfonts}
usepackage{algorithmic}
usepackage{graphicx}
usepackage{textcomp}
usepackage{xcolor}
usepackage[numbers,square,comma]{natbib}
usepackage{mfirstuc}
%usepackage[useprefix=false,backend=natbib]{biblatex}
defBibTeX{{rm Bkern-.05em{sc ikern-.025em b}kern-.08em
    Tkern-.1667emlower.7exhbox{E}kern-.125emX}}

begin{document}
capitalisewords{Citeauthor{10.1145/3196321.3196342}}

bibliographystyle{unsrtnat}
bibliography{refs} 
end{document}

Here is my refs.bib

@inproceedings{10.1145/3196321.3196342,
author = {dos Santos, Rodrigo Magalh~{a}es and Gerosa, Marco Aur'{e}lio},
title = {Impacts of Coding Practices on Readability},
year = {2018},
isbn = {9781450357142},
publisher = {Association for Computing Machinery},
address = {New York, NY, USA},
url = {https://doi.org/10.1145/3196321.3196342},
doi = {10.1145/3196321.3196342},
abstract = {Several conventions and standards aim to improve maintainability of software code. However, low levels of code readability perceived by developers still represent a barrier to their daily work. In this paper, we describe a survey that assessed the impact of a set of Java coding practices on the readability perceived by software developers. While some practices promoted an enhancement of readability, others did not show statistically significant effects. Interestingly, one of the practices worsened the readability. Our results may help to identify coding conventions with a positive impact on readability and, thus, guide the creation of coding standards.},
booktitle = {Proceedings of the 26th Conference on Program Comprehension},
pages = {277–285},
numpages = {9},
keywords = {code comprehension, software developers' opinions survey, code readability, programming style, coding best practices},
location = {Gothenburg, Sweden},
series = {ICPC '18}
}

Everytime I try to compile this with pdfLatex, I get the following

! Argument of @citex has an extra }.
<inserted text> 
                par 
l.59 ...ords{Citeauthor{10.1145/3196321.3196342}}

I have no clue how to solve this and cannot find a solution anywhere.

begin{thebibliography}{1}

bibitem{DosSantos}
Dos Santos, Rodrigo Magalh~{a}es and Gerosa, Marco Aur'{e}lio, "Impacts of Coding Practices on Readability",  emph{In Proceedings of the 26th Conference on Program Comprehension (ICPC '18). Association for Computing Machinery, New York, NY, USA, 277–285. DOI:https://doi.org/10.1145/3196321.3196342}
end{thebibliography}

Random text Dos {em et al.} cite{DosSantos}