Space before exponent subscript, superscript: Got group of unknown type: 'internal'

[nschloe]

Putting an explicit horizontal space right before exponentiation ^ is valid in LaTeX

\[
a^t
a\,^t
a\:^t
a\>^t
a\;^t
a\quad^t
a\qquad^t
\]

but doesn't work in KaTeX:

KaTeX parse error: Got group of unknown type: 'internal'

Same with _.

Edit: The same error message occurs with

.\!''

[JacobBeaudry]

N.B. in the meantime, one work-around is to create an explicit group; e.g., a{\,}^t