Comparing certain objects throws TypeError: Cannot convert object to primitive value

[joshlory]

When comparing objects with certain length properties, the reporter throws when stringifying the diff. Reduced repro:

it('compare two objects', function () {
   assert.deepEqual({length: {toString: 0}}, {length: 1});
});

Expected:

AssertionError: { length: { toString: 0 } } deepEqual { length: 1 }
+ expected
- actual

 {
-  "length": {
-    "toString": 0
-  }
+  "length": 1
 }

Actual:

TypeError: Cannot convert object to primitive value
at jsonStringify (mocha/lib/utils.js:504:7)
at Object.exports.stringify (mocha/lib/utils.js:431:14)
at mocha/lib/reporters/base.js:203:37
at Array.forEach (native)
at Function.exports.list (mocha/lib/reporters/base.js:163:12)
at Spec.Base.epilogue (mocha/lib/reporters/base.js:328:10)
at Runner.emit (events.js:129:20)
at mocha/lib/runner.js:791:12
at mocha/lib/runner.js:644:9
at next (mocha/lib/runner.js:284:14)