diff --git a/lib/config/migration.js b/lib/config/migration.js
index 4d14bac104136422c210ca3cf2c8481568cf564c..e152def7f93082bc9b9b8e7440682bba4657ef7b 100644
--- a/lib/config/migration.js
+++ b/lib/config/migration.js
@@ -233,7 +233,7 @@ function migrateConfig(config) {
         delete migratedConfig.baseBranch;
       } else if (key === 'schedule' && val) {
         // massage to array first
-        const schedules = is.string(val) ? [val] : val;
+        const schedules = is.string(val) ? [val] : [...val];
         // split 'and'
         const schedulesLength = schedules.length;
         for (let i = 0; i < schedulesLength; i += 1) {