diff --git a/lib/sandbox.js b/lib/sandbox.js
index 560f596..d91224b 100644
--- a/lib/sandbox.js
+++ b/lib/sandbox.js
@@ -285,9 +285,11 @@ function errorToFault(err) {
   };
 
   if (err.stack) {
+    fault.detail.stack = err.stack;
     // Trim the stack trace at the self-executing anonymous wrapper function
-    let stackTrimIndex = err.stack.match(/\s+at\s[^\s]+\s+at\s[^\s]+\s\(vm\.js.+\)/).index;
-    fault.detail.stack = err.stack.slice(0, stackTrimIndex);
+    let stackTrimIndex = fault.detail.stack.match(/\s+at\s[^\s]+\s+at\s[^\s]+\s\(vm\.js.+\)/);
+    if (stackTrimIndex)
+      fault.detail.stack = fault.detail.stack.slice(0, stackTrimIndex.index);
   }
 
   return fault;