diff --git a/src/core/meshcop/dtls.cpp b/src/core/meshcop/dtls.cpp
index ccf84170f..da87bf350 100644
--- a/src/core/meshcop/dtls.cpp
+++ b/src/core/meshcop/dtls.cpp
@@ -66,6 +66,7 @@ ThreadError Dtls::Start(bool aClient, ReceiveHandler aReceiveHandler, SendHandle
     mSendHandler = aSendHandler;
     mContext = aContext;
     mClient = aClient;
+    mReceiveMessage = NULL;
 
     mbedtls_ssl_init(&mSsl);
     mbedtls_ssl_config_init(&mConf);