Enotices on export when relationship type using 'any contact type' exists

If you create a relationship type and choose 'any contact type' for the ends of the relationship then when exporting search results you get a bunch of eNotices like

Notice: Undefined index: in CRM_Core_BAO_Mapping::buildMappingForm() (line 621 of /.../civicrm/CRM/Core/BAO/Mapping.php).

The line in question is
else

{ $target_type = 'contact_type_' . $second; $relatedFields = array_merge((array) $relatedMapperFields[$relationshipType->$target_type], (array) $relationshipCustomFields); }