diff --git a/packages/core/src/skeleton.js b/packages/core/src/skeleton.js
index 0c9302c..2eed5d4 100644
--- a/packages/core/src/skeleton.js
+++ b/packages/core/src/skeleton.js
@@ -378,7 +378,7 @@ Index.prototype.sendMessage = function(v, leafCallback) {
 
 Node.prototype._debugString = function (outerIndent) {
   const pieces = [];
-  const inspect = require('util').inspect || JSON.stringify;
+  const inspect = require('util').inspect;
   function line(indent, content) {
     pieces.push(indent);
     pieces.push(content);
diff --git a/packages/core/src/util_stub.js b/packages/core/src/util_stub.js
index 7c83e1f..cc32ee0 100644
--- a/packages/core/src/util_stub.js
+++ b/packages/core/src/util_stub.js
@@ -1,3 +1,11 @@
 module.exports = {
-  inspect: void 0,
+  inspect: (item) => {
+    try {
+      return JSON.stringify(item);
+    } catch (_e) {
+      return '<uninspectable_value>';
+    }
+  },
 };
+
+module.exports.inspect.custom = Symbol('util_stub.inspect.custom');