From 0f82e2a84dfef85329655e62861057795440ddca Mon Sep 17 00:00:00 2001
From: Paul Kaplan <pkaplan@media.mit.edu>
Date: Thu, 14 Dec 2017 14:21:57 -0500
Subject: [PATCH] Be more cautious about parsing json.

---
 src/engine/sequencer.js | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/src/engine/sequencer.js b/src/engine/sequencer.js
index faa40b9e8..2db1c6ed8 100644
--- a/src/engine/sequencer.js
+++ b/src/engine/sequencer.js
@@ -316,7 +316,15 @@ class Sequencer {
             const definitionBlock = thread.target.blocks.getBlock(definition);
             const innerBlock = thread.target.blocks.getBlock(
                 definitionBlock.inputs.custom_block.block);
-            const doWarp = JSON.parse(innerBlock.mutation.warp);
+            let doWarp = false;
+            if (innerBlock && innerBlock.mutation) {
+                const warp = innerBlock.mutation.warp;
+                if (typeof warp === 'boolean') {
+                    doWarp = warp;
+                } else if (typeof warp === 'string') {
+                    doWarp = JSON.parse(warp);
+                }
+            }
             if (doWarp) {
                 thread.peekStackFrame().warpMode = true;
             } else if (isRecursive) {