diff --git a/lib/reporters/base.js b/lib/reporters/base.js
index 9243ec42e6..d8ee73358e 100644
--- a/lib/reporters/base.js
+++ b/lib/reporters/base.js
@@ -155,6 +155,10 @@ exports.cursor = {
   }
 };
 
+function showDiff(err) {
+  return err && err.showDiff !== false && sameType(err.actual, err.expected) && err.expected !== undefined;
+}
+
 /**
  * Output the given `failures` as a list.
  *
@@ -183,8 +187,6 @@ exports.list = function (failures) {
     }
     var stack = err.stack || message;
     var index = message ? stack.indexOf(message) : -1;
-    var actual = err.actual;
-    var expected = err.expected;
     var escape = true;
 
     if (index === -1) {
@@ -201,13 +203,8 @@ exports.list = function (failures) {
       msg = 'Uncaught ' + msg;
     }
     // explicitly show diff
-    if (err.showDiff !== false && sameType(actual, expected) && expected !== undefined) {
+    if (showDiff(err)) {
       escape = false;
-      if (!(utils.isString(actual) && utils.isString(expected))) {
-        err.actual = actual = utils.stringify(actual);
-        err.expected = expected = utils.stringify(expected);
-      }
-
       fmt = color('error title', '  %s) %s:\n%s') + color('error stack', '\n%s\n');
       var match = message.match(/^([^:]+): expected/);
       msg = '\n      ' + color('error message', match ? match[1] : msg);
@@ -292,6 +289,12 @@ function Base (runner) {
   runner.on('fail', function (test, err) {
     stats.failures = stats.failures || 0;
     stats.failures++;
+    if (showDiff(err)) {
+      if (!utils.isString(err.actual) || !utils.isString(err.expected)) {
+        err.actual = utils.stringify(err.actual);
+        err.expected = utils.stringify(err.expected);
+      }
+    }
     test.err = err;
     failures.push(test);
   });
diff --git a/test/reporters/list.spec.js b/test/reporters/list.spec.js
index 4b3704204d..d83d4e48cc 100644
--- a/test/reporters/list.spec.js
+++ b/test/reporters/list.spec.js
@@ -192,6 +192,28 @@ describe('List reporter', function () {
 
       Base.cursor = cachedCursor;
     });
+    it('should immediately construct fail strings', function() {
+      var actual = { a: 'actual' };
+      var expected = { a: 'expected' };
+      var test = {};
+      var checked = false;
+      var err;
+      runner.on = function (event, callback) {
+        if (!checked && event === 'fail') {
+          err = new Error('fake failure object with actual/expected');
+          err.actual = actual;
+          err.expected = expected;
+          err.showDiff = true;
+          callback(test, err);
+          checked = true;
+        }
+      };
+      List.call({epilogue: function () {}}, runner);
+
+      process.stdout.write = stdoutWrite;
+      expect(typeof err.actual).to.equal('string');
+      expect(typeof err.expected).to.equal('string');
+    });
   });
 
   describe('on end', function () {