No thumbnail images in Google maps

I have Google maps displaying on my listings category view. When I click a map marker the expected info box pops up with it is missing the thumbnail image. Please see attached.

The title and description of the listing show up just fine. Looking at the code with developer tools in chrome I see that the image url for this element has been set to ‘null’:

<div class="col-md-4 media" style="background-image:url(null); background-size: cover; "><div class="ficons"><a href="javascript:void(0);" onclick="MapDrawLine(23);"><i class="fa fa-car"></i></a></div></div>

Any ideas?