Error on review form after _entityreference_usermerge_get_referencing_entity_types() returns NULL

At admin/people/merge after selecting two users to merge, I get "unexpected error" which in watchdog looks like this:

Recoverable fatal error: Object of class stdClass could not be converted to string in filter_xss() (line 1440 of [mypath]/includes/common.inc).

I know that's in common.inc but it only happens at admin/people/merge

That said, filter_xss doesn't appear to be called or overridden anywhere in usermerge. How's that for strange?

Prior to that (but still on admin/people/merge) there are seven each of:

Warning: strlen() expects parameter 1 to be string, object given in drupal_validate_utf8() (line 1594 of [mypath]/includes/bootstrap.inc).

Notice: Array to string conversion in format_string() (line 1546 of [mypath]/includes/bootstrap.inc).

and one more (the very first message generated by this page):

Warning: Invalid argument supplied for foreach() in entityreference_usermerge_build_review_form_elements() (line 17 of [mypath]/sites/all/modules/usermerge/includes/entityreference.usermerge.inc).

It looks like _entityreference_usermerge_get_referencing_entity_types() is returning something other than an array. I did a var_dump on it and it is coming back NULL. I'm churning through why that might be but I don't have an immediate answer. Doesn't seem like expected behavior though.

Any ideas?