diff --git a/include/serial/serial_listener.h b/include/serial/serial_listener.h
index 022623b..7e85b4c 100644
--- a/include/serial/serial_listener.h
+++ b/include/serial/serial_listener.h
@@ -159,6 +159,12 @@ public:
 
   ComparatorType comparator_;
   DataCallback callback_;
+
+private:
+  // Disable copy constructors
+  Filter(const Filter&);
+  void operator=(const Filter&);
+  const Filter& operator=(Filter);
 };
 
 /*!
@@ -173,7 +179,7 @@ public:
  */
 typedef boost::shared_ptr<Filter> FilterPtr;
 
-class BlockingFilter;
+class BlockingFilter; // Forward declaration
 
 /*!
  * Shared Pointer of BlockingFilter, returned by
@@ -183,7 +189,7 @@ class BlockingFilter;
  */
 typedef boost::shared_ptr<BlockingFilter> BlockingFilterPtr;
 
-class BufferedFilter;
+class BufferedFilter; // Forward declaration
 
 /*!
  * Shared Pointer of BufferedFilter, returned by
@@ -637,6 +643,10 @@ public:
   }
 
 private:
+  // Disable copy constructors
+  SerialListener(const SerialListener&);
+  void operator=(const SerialListener&);
+  const SerialListener& operator=(SerialListener);
   // delimeter tokenizer function
   static void
   _delimeter_tokenizer (const std::string &data,
diff --git a/src/serial_listener.cc b/src/serial_listener.cc
index bc1258d..a7bbe98 100644
--- a/src/serial_listener.cc
+++ b/src/serial_listener.cc
@@ -52,7 +52,9 @@ SerialListener::callback() {
       if (this->callback_queue.timed_wait_and_pop(pair, 10)) {
         if (this->listening) {
           try {
-            pair.first->callback_((*pair.second));
+            if (pair.first != NULL && pair.second != NULL) {
+              pair.first->callback_((*pair.second));
+            }
           } catch (std::exception &e) {
             this->handle_exc(e);
           }// try callback