Damn - sorry, misremembered the issue. Lein solved a different problem I had 
The solution to this issue for me was to make sure figwheel-sidecar and cider/piggieback and devtools were the latest versions. I changed the dev dependencies in project.clj to these:
[figwheel-sidecar “0.5.18”]
[cider/piggieback “0.4.1-SNAPSHOT”]
[binaryage/devtools “0.9.10”]