diff --git a/lib/roles/provider.js b/lib/roles/provider.js
index 10d99e45..a28e1d95 100644
--- a/lib/roles/provider.js
+++ b/lib/roles/provider.js
@@ -212,7 +212,8 @@ module.exports = function(options) {
         return worker = workers[device.id] = spawn()
           .then(function() {
             log.info('Device worker "%s" has retired', device.id)
-            worker = workers[device.id] = null
+            delete workers[device.id]
+            worker = null
           })
           .catch(procutil.ExitError, function(err) {
             log.error(