How to fix error Missing number, treated as zero

Question

I know this must be a common title for a question, and I have done my research but I still can't figure out why i get this error with this block of code:

\documentclass{sig-alternate}
\usepackage[british]{babel}
\usepackage{fancyhdr}
\usepackage{booktabs}
\usepackage[utf8]{inputenc}
\usepackage{libertine}
\usepackage{balance}
\usepackage{hyperref}
\usepackage{url}
\graphicspath{ {./images/} }

\setlength{\heavyrulewidth}{1.2pt}
\setlength{\lightrulewidth}{0.7pt}
\pagestyle{fancy}
\lhead{UMAP 2017 Short Paper}
\rhead{UMAP’17, July 9-12, 2017, Bratislava, Slovakia}

\renewcommand{\headrulewidth}{}
\renewcommand{\footrulewidth}{}
\newfont{\mycrnotice}{ptmr8t at 7pt}
\newfont{\myconfname}{ptmri8t at 7pt}
\let\crnotice\mycrnotice
\let\confname\myconfname

\begin{document}
\title{Improving Cold Start Recommendation by Mapping
        Feature-Based Preferences to Item Comparisons}

\numberofauthors{2}

\author{
\alignauthor
Saikishore Kalloori\\
       \affaddr{Free University of Bozen - Bolzano,}\\
       \affaddr{Piazza Domenicani 3, I - 39100,}\\
       \email{[email protected]}
\alignauthor
Francesco Ricci\\
       \affaddr{Free University of Bozen - Bolzano,}\\
       \affaddr{Piazza Domenicani 3, I - 39100,}\\
       \email{[email protected]}
}
\maketitle
\section{EXPERIMENTAL EVALUATION}\noindent
\subsection{Data Set}\noindent
In order to test our hypothesis we needed a dataset containing both
item and feature preferences. The only publicly available data set
with such preferences, which we could identify, is the PoliMovie
dataset \cite{10}. For each user in the dataset, there are preferences
expressed explicitly for movies and for their features, in the form
of likes.
\bibliographystyle{abbrv}
\bibliography{zdroje}
\end{document}

Error is pointed to the first line with \section. It says:

Missing number, treated as zero. Overfull \hbox(1.82866 pt too wide) in paragraph at lines 140--154.

Link to sig-alternate and my .bib file: https://drive.google.com/open?id=1Hg5Rw9iFLI1w6hSyAwlRyX29bkMwyIXY

I have couple of errors like this scattered throughout the whole document near \section commands. I see only couple of lines highlighted with this error(no other kinds of errors appear) but overleaf says that I have 37 errors, even though the document compiles OK, and looks just like it's supposed to look. Any ideas how to fix this error? Thank you for your help and time.

Answer 1

The answer was posted by user @egreg, I just needed to change commands:

\renewcommand\headrulewidth{} 
\renewcommand{\footrulewidth}{} 

to:

\renewcommand\headrulewidth{0pt}
\renewcommand{\footrulewidth}{0pt}

The issue was connected with package fancyhdr,as user @moewe pointed out. Thank you all for your help.