1 files changed, 4 insertions, 1 deletions
diff --git a/intern/cycles/session/session.cpp b/intern/cycles/session/session.cpp
index f6e06f20aba..8fcb5c9ac96 100644
--- a/intern/cycles/session/session.cpp
+++ b/intern/cycles/session/session.cpp
@@ -110,7 +110,10 @@ void Session::start()
   {
     /* Signal session thread to start rendering. */
     thread_scoped_lock session_thread_lock(session_thread_mutex_);
-    assert(session_thread_state_ == SESSION_THREAD_WAIT);
+    if (session_thread_state_ == SESSION_THREAD_RENDER) {
+      /* Already rendering, nothing to do. */
+      return;
+    }
     session_thread_state_ = SESSION_THREAD_RENDER;
   }