From 7cc0c00da66dc1378b6069b99f04bd575f35064e Mon Sep 17 00:00:00 2001
From: Tim Mickel <tim.mickel@gmail.com>
Date: Fri, 10 Jun 2016 08:47:54 -0400
Subject: [PATCH] Correctly deal with empty substacks.

The stack needs to be popped in this situation.
---
 src/engine/sequencer.js | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/src/engine/sequencer.js b/src/engine/sequencer.js
index cbccd09b..1593c190 100644
--- a/src/engine/sequencer.js
+++ b/src/engine/sequencer.js
@@ -81,7 +81,9 @@ Sequencer.prototype.stepThreads = function (threads) {
  */
 Sequencer.prototype.startThread = function (thread) {
     var currentBlockId = thread.peekStack();
-    if (!currentBlockId || !this.runtime.blocks.getBlock(currentBlockId)) {
+    if (!currentBlockId) {
+        // A "null block" - empty substack. Pop the stack.
+        thread.popStack();
         thread.status = Thread.STATUS_DONE;
         return;
     }