Empty png and HTML table when creating HTML file from man pages

[mobimik]

I try to use groff for convert man pages to HTML with this command:
zcat /usr/share/man/man3/exit.3.gz   groff -mandoc -Thtml

This generates a empty png file (zero size)!

Image excepted is in fact a table (man 3 exit):
┌──────────┬───────────────┬─────────────────────┐
│Interface │ Attribute     │ Value               │
├──────────┼───────────────┼─────────────────────┤
│exit()    │ Thread safety │ MT-Unsafe race:exit │
└──────────┴───────────────┴─────────────────────┘

Groff also generates HTML code for the table, but it is empty!
<table width="100%" border="0" rules="none" frame="void"
       cellspacing="0" cellpadding="0">
<tr valign="top" align="left">
<td width="8%"></td>
<td width="7%">
</td>
<td width="8%">
</td>
<td width="8%"></td>
<td width="7%"></td>
<td width="62%">
</td></tr>
<tr valign="top" align="left">
<td width="8%"></td>
<td width="7%"></td>
<td width="8%">
</td>
<td width="8%"></td>
<td width="7%"></td>
<td width="62%">
</td></tr>
</table>

Is it normal or bug?