Tags not work after deleting related value

[hershs]

The flow:

Crete two ephemerals "/mydata/eph1" and "/mydata/eph2":
PUT http://my_host:5678/ephemerals/mydata/eph1
data1
PUT http://my_host:5678/ephemerals/mydata/eph2
data2

relate it to tag "mytag":
PUT http://my_host:5678/ephemerals/mydata/eph1/tag
{"tags":"mytag"}
PUT http://my_host:5678/ephemerals/mydata/eph2/tag
{"tags":"mytag"}

Currently I can get all ephemerals by tag by:
GET http://my_host:5678/tags/mytag

Delete one of ephemerals by
DELETE http://my_host:5678/ephemerals/mydata/eph1

Now, when I try to get it by tag again I got error:
{"result":"failure","error_message":"undefined method `name' for nil:NilClass"}