Issue in title of frame during allowframebreaks

Question:

When I use allowframebreaks in bibliography, the title of the slide is changed on the next page (on the first slide Reference I and on the second slide Reference II). How did this happen?

What I have so far:

documentclass[12pt]{beamer}
usepackage{xcolor}
usetheme{AnnArbor}
usepackage[f]{esvect}

usecolortheme{beaver}

usefonttheme{professionalfonts} % using non standard fonts for beamer
usefonttheme{serif} % default family is serif
usepackage{setspace}


begin{document}
begin{frame}[t,allowframebreaks]{textbf{References}}
    begin{thebibliography}{10}
        setbeamertemplate{bibliography item}[text]
    bibitem{oberbeck1879warmeleitung}
    alert{{"U}ber die W{"a}rmeleitung der Fl{"u}ssigkeiten bei Ber{"u}cksichtigung der Str{"o}mungen infolge von Temperaturdifferenzen}
    newblock {Oberbeck, Anton}
    newblock {Wiley Online Library 1879}.

    bibitem{oberbeck1888uber}
    alert{Uber die Bewegungsercheinungen der Atmosphere. Sitz. Ber. K}
    newblock {Oberbeck, A}
    newblock {Preuss. Akad. Miss 1888}.

    bibitem{boussinesq1897theorie}
    alert{Boussinesq, Joseph}
    newblock  {Th{'e}orie de l'{'e}coulement tourbillonnant et tumultueux des liquides dans les lits rectilignes a grande section}
    newblock {Gauthier-Villars 1897}.

    bibitem{bachelier1900theorie}
    alert{Bachelier, Louis}
    newblock {Annales scientifiques de l'{'E}cole normale sup{'e}rieure}
    newblock {1900}.

    bibitem{struve1931astrophysik}
    alert{Struve, Otto}
    newblock {The Astrophysical Journal}
    newblock {1931}.

        end{thebibliography}
end{frame}
end{document}