Sorting problem with umlauts

Question

I have a problem with a self-generated index of authors. If I have a name with umlaut, the sorting fails. In My MWE, the two entries should be inverted. How could I fix the issue? I'm including also the coding for the "namea" mod, in case it could be useful.

% !BIB TS-program = biber
% !BIB program = biber
% !TEX encoding = UTF-8 Unicode
% !TeX TS-program = xelatexmk

\begin{filecontents}{archivio.bib}

@book{Doll,
    Address = {Moskwa},
    Author = {Pippo Donninger},
    Publisher = {Sinodal'naja tip.},
    Title = {I'm the sheep},
    Year = {1894}}

@book{Dol2,
    Address = {Moskwa},
    Author = {Philip Döllinger},
    Publisher = {Sinodal'naja tip.},
    Title = {Sistematičeskoe opisanie rukopisej moskovskoj Sinodal'noj (patriaršej) biblioteki},
    Year = {1894}}


\end{filecontents}

\documentclass[11pt, openany]{book}
\usepackage{polyglossia}
\setmainlanguage[babelshorthands=true]{italian}
\setotherlanguages{latin, english, french}

\usepackage[]{csquotes}
\usepackage{xpatch}
\usepackage[style=verbose-trad2,
            language=auto,
            ibidpage=true,
            autolang=other,
            useprefix=true,
            giveninits=true,
            indexing=true,
            dateabbrev=false,
            backend=biber,
        ]{biblatex}



\addbibresource{archivio.bib}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%


\usepackage{imakeidx}
\makeindex[name=nomi, intoc=true, title=Indice dei nomi]

\renewbibmacro*{citeindex}{%
  \ifciteindex
    {\ifnameundef{namea}{\indexnames{labelname}}{\indexnames{namea}}}
    {}}

\DeclareIndexNameFormat{default}{%      Formato del nome dell'autore
  \usebibmacro{index:name}
    {\index[nomi]}
    {\namepartfamily}
    {\namepartgiveni}
    {\namepartprefix}
    {\namepartsuffix}}


\begin{document}


I should like to sort properly \cite{Doll} and \cite{Dol2}.



\printindex[nomi]       % Indice dei nomi   
\end{document}