(update-in response [?key] json/generate-string)
It appears that [?key] is a placeholder that did not resolve correctly.
I think this is related to http://dev.clojure.org/jira/browse/LOGIC-118
LOGIC-118 has been closed, a new core.logic release should be coming in a little bit.
Update core.logic to 0.8.10
Fixes #77, closes #116