diff --git a/src/osgViewer/Viewer.cpp b/src/osgViewer/Viewer.cpp
index d4f5fe5eb..569f22434 100644
--- a/src/osgViewer/Viewer.cpp
+++ b/src/osgViewer/Viewer.cpp
@@ -1004,6 +1004,8 @@ void Viewer::getViews(Views& views, bool onlyValid)
 
 void Viewer::getAllThreads(Threads& threads, bool onlyActive)
 {
+    threads.clear();
+
     OperationThreads operationThreads;
     getOperationThreads(operationThreads);