eqref and cases package

I want to use a custom tag for some equations, but I’m not being allowed to. Here is an MWE:

documentclass{article}

usepackage{amsmath,cases}

begin{document}
    begin{equation}
        x=ytag{custom1}label{eq:1}
    end{equation}
    begin{numcases}{}
        x=y tag{custom2}label{eq:2}
    end{numcases}
    Why can I do eqref{eq:1}, but not eqref{eq:2}?
end{document}

This produces the following output:

Naturally I’d like the second eqref to display as (custom2).