--- a/services/sync/modules/FxaMigrator.jsm
+++ b/services/sync/modules/FxaMigrator.jsm
@@ -196,16 +196,17 @@ Migrator.prototype = {
 
     // Write the migration sentinel if necessary.
     yield this._setMigrationSentinelIfNecessary();
 
     // Must be ready to perform the actual migration.
     this.log.info("Performing final sync migration steps");
     // Do the actual migration.
     let startOverComplete = new Promise((resolve, reject) => {
+      let observe;
       Services.obs.addObserver(observe = () => {
         this.log.info("observed that startOver is complete");
         Services.obs.removeObserver(observe, "weave:service:start-over:finish");
         resolve();
       }, "weave:service:start-over:finish", false);
     });
 
     Weave.Service.startOver();