diff --git a/src/AsyncWebSocket.cpp b/src/AsyncWebSocket.cpp
index 9deb74a..aeb3747 100644
--- a/src/AsyncWebSocket.cpp
+++ b/src/AsyncWebSocket.cpp
@@ -595,7 +595,7 @@ void AsyncWebSocketClient::_onData(void* pbuf, size_t plen) {
     }
 
     // restore byte as _handleEvent may have added a null terminator i.e., data[len] = 0;
-    if (datalen)
+    if (data && datalen > 0)
       data[datalen] = datalast;
 
     data += datalen;