siunitx error: "invalid-number" Invalid numerical input 'e'

documentclass{article}

begin{document}
begin{table}[]
    centering
    caption{Test table}
    begin{tabular}{|l|c|c|c|c|c|c|}
        hline
        & multicolumn{5}{c|}{Numbers 1 [si{millivolt}]} & Mean $pm$ max. Variance  [si{millivolt}] \ hline hline
        P1        & -241    & -274    & -272    & -258    & -283   & -266                           \ hline
        P2        & -94     & -99     & -100    & -114    & -105   & -102                           \ hline
        P3        & 600     & 593     & 595     & 580     & 590    & 511                            \ hline
        P4        & 300     & 300     & 300     & 290     & 297    & 301                            \ hline
        P5        & -730    & -734    & -732    & -778    & -741   & -738                           \ hline
    end{tabular}
end{table}
end{document}

documentclass{article}
usepackage{siunitx}
begin{document}
begin{table}
    centering
    caption{Test tablestrut} % use 'strut' to create a bit more vert. separation
    begin{tabular}{|l|*{6}{S[table-format=-3.0]|}}
    hline
    & multicolumn{5}{c|}{Numbers 1 [si{millivolt}]}
    & {Mean $pm$ max. Variance  [si{millivolt}]} \
    hline hline
    P1        & -241    & -274    & -272    & -258    & -283   & -266 \ hline
    P2        &  -94    &  -99    & -100    & -114    & -105   & -102 \ hline
    P3        &  600    &  593    &  595    & 580     &  590   &  511 \ hline
    P4        &  300    &  300    &  300    & 290     &  297   &  301 \ hline
    P5        & -730    & -734    & -732    & -778    & -741   & -738 \ hline
    end{tabular}
end{table}
end{document}