Missing = inserted for \ifnum error when generating bibliography using natbib

Question

I'm trying to insert citations and a bibliography in ASA citation style using natbib. I'm trying to adapt a latex file that a friend sent me, since generating bibliographies is new to me. The bibliography is generated perfectly well but all of the citations show up as question marks and the console prints the error: Missing = inserted for \ifnum

Sample code:

\documentclass[letterpaper,12pt,openany]{memoir}
\usepackage{natbib} % ASA citation style

\begin{document}

\chapter{Introduction}
\thispagestyle{empty}

This is the introduction. \citep{Rubin1974}

%% Bibliography
\bibliographystyle{asa}
\bibliography{testbibliography}

\end{document}

My testbibliography document is the following:

%% This BibTeX bibliography file was created using BibDesk.
%% http://bibdesk.sourceforge.net/

%% Saved with string encoding Unicode (UTF-8) 

@article{Rubin1974,
    Author = {B. Rubin, Donald},
    Booktitle = {Journal of Educational Psychology},
    Date-Added = {2018-02-02 05:35:05 +0000},
    Date-Modified = {2018-02-02 16:48:13 +0000},
    Journal = {Journal of Educational Psychology},
    Month = {10},
    Title = {Estimating causal effects of treatments in randomized and nonrandomized studies},
    Volume = {66},
    Year = {1974}}

Additionally, I downloaded the files asa.bst and asa.sty from this site, since I was initially getting errors about missing these files.

I keep getting the following error:

This is pdfTeX, Version 3.14159265-2.6-1.40.16 (TeX Live 2015) (preloaded format=pdflatex)
 \write18 enabled.
entering extended mode
(./bibliography_test.tex
LaTeX2e <2015/01/01>
Babel <3.9l> and hyphenation patterns for 79 languages loaded.
(/usr/local/texlive/2015/texmf-dist/tex/latex/memoir/memoir.cls
Document Class: memoir 2015/04/23 v3.7d configurable book, report, article docu
ment class
(/usr/local/texlive/2015/texmf-dist/tex/generic/oberdiek/ifpdf.sty)
(/usr/local/texlive/2015/texmf-dist/tex/latex/ifetex/ifetex.sty
(/usr/local/texlive/2015/texmf-dist/tex/plain/ifetex/ifetex.tex))
(/usr/local/texlive/2015/texmf-dist/tex/generic/ifxetex/ifxetex.sty)
(/usr/local/texlive/2015/texmf-dist/tex/generic/oberdiek/ifluatex.sty)
(/usr/local/texlive/2015/texmf-dist/tex/latex/etex-pkg/etex.sty)
(/usr/local/texlive/2015/texmf-dist/tex/latex/memoir/mem12.clo)
(/usr/local/texlive/2015/texmf-dist/tex/latex/memoir/mempatch.sty))
(/usr/local/texlive/2015/texmf-dist/tex/latex/natbib/natbib.sty)
(./bibliography_test.aux)
./bibliography_test.tex:9: Missing = inserted for \ifnum.
<to be read again> 
                   \def 
l.9 This is the introduction. \citep{Rubin1974}

In other words, I always get this error when I try to insert citations (Missing = inserted for \ifnum). However, the bibliography itself is fine and any article that I try to cite will show up appropriately in the bibliography. However, the citations themselves always show up as question marks. Any ideas what is going wrong here?