Cannot use subtitle

Subtitles cannot be added even after the subtitle and subtitlefi commands were introduced within !4 (merged). The reason is that the \abst@subtitle command is not defined.

Trying to add a subtitle raises Undefined control sequence error.

! Undefined control sequence.
<argument> ...\@subtitle \noindent \abst@subtitle
                                                  \@subtitle \fi
l.105 \maketitle