diff --git a/lib/index.js b/lib/index.js
index 336d8d2..a3f61d6 100644
--- a/lib/index.js
+++ b/lib/index.js
@@ -165,6 +165,10 @@ export async function downloadUpdate(options, eventListeners) {
     logger(`duplicated downloaded hash ${downloadedHash}, ignored`);
     return;
   }
+  if (readyHash) {
+    logger(`hash ${readyHash} applied. reboot first`);
+    return;
+  }
   if (downloadingThrottling) {
     logger('repeated downloading, ignored');
     return;
@@ -215,21 +219,34 @@ export async function downloadUpdate(options, eventListeners) {
   return options.hash;
 }
 
-export function switchVersion(hash) {
-  assertRelease();
-  logger('switchVersion');
-  Pushy.reloadUpdate({ hash });
-}
-
 let readyHash;
-export function switchVersionLater(hash) {
-  assertRelease();
+function assertHash(hash) {
+  if (!downloadedHash) {
+    logger(`no downloaded hash`);
+    return;
+  }
+  if (hash !== downloadedHash) {
+    logger(`use downloaded hash ${downloadedHash} first`);
+    return;
+  }
   if (readyHash === hash) {
-    logger(`duplicated ready hash ${readyHash}, ignored`);
+    logger(`hash ${readyHash} already applied. reboot first.`);
     return;
   }
   readyHash = hash;
-  logger('switchVersionLater');
+}
+
+export function switchVersion(hash) {
+  assertRelease();
+  assertHash(hash);
+  logger('switchVersion: ' + hash);
+  Pushy.reloadUpdate({ hash });
+}
+
+export function switchVersionLater(hash) {
+  assertRelease();
+  assertHash(hash);
+  logger('switchVersionLater: ' + hash);
   Pushy.setNeedUpdate({ hash });
 }