diff --git a/RenderView.cpp b/RenderView.cpp
index 4d9edc3..f5e57e0 100644
--- a/RenderView.cpp
+++ b/RenderView.cpp
@@ -75,7 +75,12 @@ void RenderView::draw()
         scene.root()->draw(glm::identity<glm::mat4>(), P);
         fading_overlay_->draw(glm::identity<glm::mat4>(), projection);
         frame_buffer_->end();
+    }
+}
 
+void RenderView::drawThumbnail()
+{
+    if (frame_buffer_) {
         // if a thumbnailer is pending
         if (thumbnailer_.size() > 0) {
 
@@ -97,14 +102,14 @@ void RenderView::draw()
                 }
 
                 // return valid thumbnail promise
-                thumbnailer_.front().set_value( frame_thumbnail->image() );
+                thumbnailer_.back().set_value( frame_thumbnail->image() );
 
                 // done with thumbnailing framebuffer
                 delete frame_thumbnail;
             }
             catch(...) {
                 // return failed thumbnail promise
-                thumbnailer_.front().set_exception(std::current_exception());
+                thumbnailer_.back().set_exception(std::current_exception());
             }
 
             // done with this promise
@@ -113,7 +118,6 @@ void RenderView::draw()
     }
 }
 
-
 FrameBufferImage *RenderView::thumbnail ()
 {
     // by default null image
diff --git a/RenderView.h b/RenderView.h
index cf3d38e..7a2a7f1 100644
--- a/RenderView.h
+++ b/RenderView.h
@@ -19,6 +19,7 @@ public:
     RenderView ();
     ~RenderView ();
 
+    // render frame (in opengl context)
     void draw () override;
     bool canSelect(Source *) override { return false; }
 
@@ -28,9 +29,11 @@ public:
     void setFading(float f = 0.f);
     float fading() const;
 
+    // current frame
     inline FrameBuffer *frame () const { return frame_buffer_; }
 
-    // get a thumbnail; wait for a promise to be fullfiled
+    // get a thumbnail outside of opengl context; wait for a promise to be fullfiled after draw
+    void drawThumbnail();
     FrameBufferImage *thumbnail ();
 };
 
diff --git a/Session.cpp b/Session.cpp
index 23ff798..c7a7606 100644
--- a/Session.cpp
+++ b/Session.cpp
@@ -87,6 +87,7 @@ void Session::update(float dt)
 
     // pre-render of all sources
     failedSource_ = nullptr;
+    bool ready = true;
     for( SourceList::iterator it = sources_.begin(); it != sources_.end(); ++it){
 
         // ensure the RenderSource is rendering this session
@@ -98,6 +99,8 @@ void Session::update(float dt)
             failedSource_ = (*it);
         }
         else {
+            if ( !(*it)->ready() )
+                ready = false;
             // render the source
             (*it)->render();
             // update the source
@@ -129,6 +132,10 @@ void Session::update(float dt)
 
     // draw render view in Frame Buffer
     render_.draw();
+
+    // draw the thumbnail only after all sources are ready
+    if (ready)
+        render_.drawThumbnail();
 }
 
 SourceList::iterator Session::addSource(Source *s)