As Kovid said, the TOC is built from text it finds that could be considered a heading of some sort. If there is only the map, then there isn't anything to use. I suppose if there is an ALT text for the map, that could be used.
What I do is to add a hidden heading. The heading is:
Code:
<h1 class="hidden">Map</h1>
And the style is:
Code:
.hidden {
display:none;
}
That gets a TOC entry generated without any text shown.