diff --git a/unknow-server-nio/src/main/java/unknow/server/nio/NIOConnection.java b/unknow-server-nio/src/main/java/unknow/server/nio/NIOConnection.java
index 18982b85..331b3846 100644
--- a/unknow-server-nio/src/main/java/unknow/server/nio/NIOConnection.java
+++ b/unknow-server-nio/src/main/java/unknow/server/nio/NIOConnection.java
@@ -281,7 +281,9 @@ public synchronized void write(ByteBuffer b) throws IOException {
 		}
 
 		private synchronized void tryWrite(ByteBuffer b) throws IOException {
-			NIOConnection co = h;
+			final NIOConnection co = h;
+			if (co == null)
+				throw new IOException("already closed");
 			try {
 				while (!co.canWrite()) {
 					wait();