% An unclosed \stgroup swallows the rest of the band into its fit list; TeX
% only reports the resulting empty row, which does not point at the cause.
\documentclass{standalone}
\usepackage[en]{supertensor}
\stdim{T}{4}
\begin{document}\begin{tikzpicture}
  \strow{R1}{T}
    \stgroup{G}
      \stface{A}{}{T}{T}
  \strowend
  \sttopformula{F}{$A$}
\end{tikzpicture}\end{document}