Avoid problem with table inside table

`pandoc` convert

~~~
<table>
  <tbody>
    <tr><td valign="top"><p><strong>Person</strong>: people who took readings.</p>
    <table>
      <tbody>
        <tr><th>ident</th><th>personal</th><th>family</th></tr>
~~~

to

~~~
\begin{longtable}[c]{@{}ll@{}}
\toprule\addlinespace
\begin{minipage}[t]{0.47\columnwidth}\raggedright
\textbf{Person}: people who took readings.

\begin{longtable}[c]{@{}lll@{}}
\toprule\addlinespace
ident & personal & family
~~~

and `pdflatex` gives the follow error:

~~~
! Argument of \LT@nofcols has an extra }.
<inserted text>
                \par
l.11692 \begin{longtable}[c]{@{}lll@{}}
~~~