Package hyperref Warning: Token not allowed in a PDF string (PDFDocEncoding): (hyperref) removing `-command'

My MWE:

documentclass[12pt]{amsbook}

usepackage{amsbooka}

usepackage{hyperref}
pdfstringdefDisableCommands{%
  letenspaceempty  % this causes the warning for kern
  letnoindentempty % this causes the warning for indent
}

title{Hello}
author{Just Me}

begin{document}

maketitle
tableofcontents

part{Commutative algebra}

chapter{Some properties about the radical author{Me Again}} 

It's like, woah!

end{document}

I get the error:

Package hyperref Warning: Token not allowed in a PDF string (PDFDocEncoding):
(hyperref)                removing `<def>-command' on input line 19.

[5] [6]
Chapter 1
./mwe.tex:21: TeX capacity exceeded, sorry [input stack size=5000].
MakeUppercase #1->protect MakeUppercase  
                                            {#1}
l.21 ...rties about the radical author{Me Again}}
                                                  
./mwe.tex:21:  ==> Fatal error occurred, no output PDF file produced!
Transcript written on mwe.log.

(The lines for pdfstringdefDisableCommands suppress some of the hyperref warnings, but not the one above.)

It works if I remove the amsbooka package, but I need that for my document (to allow each chapter to have its own author). But perhaps that is a clue?