diff --git a/mailnews/base/src/nsUrlListenerManager.cpp b/mailnews/base/src/nsUrlListenerManager.cpp
index c3454f6ae80..139a3e53343 100644
--- a/mailnews/base/src/nsUrlListenerManager.cpp
+++ b/mailnews/base/src/nsUrlListenerManager.cpp
@@ -48,7 +48,7 @@ NS_IMPL_THREADSAFE_ISUPPORTS(nsUrlListenerManager, nsIUrlListenerManager::GetIID
 
 nsresult nsUrlListenerManager::RegisterListener(nsIUrlListener * aUrlListener)
 {
-	if (m_listeners)
+	if (m_listeners && aUrlListener)
 		m_listeners->AppendElement(aUrlListener);
 
 	return NS_OK;