diff --git a/src/core/coap/coap_secure.cpp b/src/core/coap/coap_secure.cpp
index 0bd90b9e9..25f5117a8 100644
--- a/src/core/coap/coap_secure.cpp
+++ b/src/core/coap/coap_secure.cpp
@@ -252,21 +252,22 @@ otError CoapSecure::HandleDtlsSend(const uint8_t *aBuf, uint16_t aLength, uint8_
         mTransmitMessage->SetLinkSecurityEnabled(false);
     }
 
+    SuccessOrExit(error = mTransmitMessage->Append(aBuf, aLength));
+
     // Set message sub type in case Joiner Finalize Response is appended to the message.
     if (aMessageSubType != Message::kSubTypeNone)
     {
         mTransmitMessage->SetSubType(aMessageSubType);
     }
 
-    VerifyOrExit(mTransmitMessage->Append(aBuf, aLength) == OT_ERROR_NONE, error = OT_ERROR_NO_BUFS);
-
     mTransmitTask.Post();
 
 exit:
 
-    if (error != OT_ERROR_NONE && mTransmitMessage != NULL)
+    if (error != OT_ERROR_NONE && mTransmitMessage != NULL && mTransmitMessage->GetLength() == 0)
     {
         mTransmitMessage->Free();
+        mTransmitMessage = NULL;
     }
 
     otLogFuncExitErr(error);
diff --git a/src/core/meshcop/commissioner.cpp b/src/core/meshcop/commissioner.cpp
index 06c6ba4c4..0ff73d4e2 100644
--- a/src/core/meshcop/commissioner.cpp
+++ b/src/core/meshcop/commissioner.cpp
@@ -1025,7 +1025,7 @@ otError Commissioner::SendRelayTransmit(Message &aMessage, const Ip6::MessageInf
     tlv.SetLength(aMessage.GetLength());
     SuccessOrExit(error = message->Append(&tlv, sizeof(tlv)));
     offset = message->GetLength();
-    message->SetLength(offset + aMessage.GetLength());
+    SuccessOrExit(error = message->SetLength(offset + aMessage.GetLength()));
     aMessage.CopyTo(0, offset, aMessage.GetLength(), *message);
 
     messageInfo.SetPeerAddr(netif.GetMle().GetMeshLocal16());