diff --git a/History.md b/History.md
index 077a135..a34ed62 100644
--- a/History.md
+++ b/History.md
@@ -1,3 +1,10 @@
+0.5.1 / 2015-06-06
+==================
+
+* bump `engine.io-client`.
+* fix timeout option
+* fix NullPointerException on ack
+
 0.5.0 / 2015-05-02
 ==================
 
diff --git a/README.md b/README.md
index 090492d..9491967 100644
--- a/README.md
+++ b/README.md
@@ -19,7 +19,7 @@ Add the following dependency to your `pom.xml`.
   <dependency>
     <groupId>com.github.nkzawa</groupId>
     <artifactId>socket.io-client</artifactId>
-    <version>0.5.0</version>
+    <version>0.5.1</version>
   </dependency>
 </dependencies>
 ```
@@ -28,7 +28,7 @@ Add the following dependency to your `pom.xml`.
 Add it as a gradle dependency for Android Studio, in `build.gradle`:
 
 ```groovy
-compile 'com.github.nkzawa:socket.io-client:0.5.0'
+compile 'com.github.nkzawa:socket.io-client:0.5.1'
 ```
 
 ## Usage