diff --git a/lib/sinon/assert.js b/lib/sinon/assert.js
index 28876673b..b8760c939 100644
--- a/lib/sinon/assert.js
+++ b/lib/sinon/assert.js
@@ -79,6 +79,7 @@ function mirrorPropAsAssertion(name, method, message) {
         var failed = false;
 
         verifyIsValidAssertion(name, args);
+
         if (typeof method === "function") {
             failed = !method(fake);
         } else {
diff --git a/lib/sinon/spy.js b/lib/sinon/spy.js
index f1f68685d..4b2b46d75 100644
--- a/lib/sinon/spy.js
+++ b/lib/sinon/spy.js
@@ -467,10 +467,10 @@ spyApi.formatters = {
             for (var j = 0; j < calledArgs.length || j < args.length; ++j) {
                 message += "\n";
                 var calledArgMessage = j < calledArgs.length ? sinonFormat(calledArgs[j]) : "";
-                var expectedArgMessage = j < args.length ? sinonFormat(args[j]) : "";
                 if (sinonMatch.isMatcher(args[j])) {
                     message += colorSinonMatchText(args[j], calledArgs[j], calledArgMessage);
                 } else {
+                    var expectedArgMessage = j < args.length ? sinonFormat(args[j]) : "";
                     var diff = jsDiff.diffJson(calledArgMessage, expectedArgMessage);
                     message += colorDiffText(diff);
                 }