diff --git a/src/harness/harness.ts b/src/harness/harness.ts
index 8f56a23994352..a65d6ac9a91f2 100644
--- a/src/harness/harness.ts
+++ b/src/harness/harness.ts
@@ -37,19 +37,20 @@ namespace assert {
         assert(!expr, msg);
     }
     export function equal<T>(a: T, b: T, msg?: string): void {
-        assert(a === b, msg || (() => `Expected to be equal:\nExpected:\n${JSON.stringify(a)}\nActual:\n${JSON.stringify(b)}`));
+        assert(a === b, () => `${msg ? msg + "\n" : ""}Expected to be equal:\nExpected:\n${stringify(a)}\nActual:\n${stringify(b)}`);
     }
     export function notEqual<T>(a: T, b: T, msg = "Expected values to not be equal."): void {
-        assert(a !== b, msg);
+        assert(a !== b, () => `${msg ? msg + "\n" : ""}Expected to not be equal:\n${JSON.stringify(a)}`);
     }
-    export function isDefined(x: {} | null | undefined, msg = "Expected value to be defined."): void {
-        assert(x !== undefined && x !== null, msg);
+    export function isDefined(x: {} | null | undefined, msg?: string): void {
+        assert(x !== undefined && x !== null, `${msg ? msg + "\n" : ""}Expected value to be defined.`);
     }
-    export function isUndefined(x: {} | null | undefined, msg = "Expected value to be undefined."): void {
-        assert(x === undefined, msg);
+    export function isUndefined(x: {} | null | undefined, msg?: string): void {
+        assert(x === undefined, () => `${msg ? msg + "\n" : ""}Expected value to be undefined:\n${stringify(x)}`);
     }
     export function deepEqual<T>(a: T, b: T, msg?: string): void {
-        assert(isDeepEqual(a, b), msg || (() => `Expected values to be deeply equal:\nExpected:\n${JSON.stringify(a, undefined, 4)}\nActual:\n${JSON.stringify(b, undefined, 4)}`));
+        assert(isDeepEqual(a, b), (() =>
+            `${msg ? msg + "\n" : ""}Expected values to be deeply equal:\nExpected:\n${stringify(a)}\nActual:\n${stringify(b)}`));
     }
     export function lengthOf(a: ReadonlyArray<{}>, length: number, msg = "Expected length to match."): void {
         assert(a.length === length, msg);
@@ -76,6 +77,10 @@ namespace assert {
         const bKeys = Object.keys(b).sort();
         return aKeys.length === bKeys.length && aKeys.every((key, i) => bKeys[i] === key && isDeepEqual((a as any)[key], (b as any)[key]));
     }
+
+    function stringify(value: {} | null | undefined): string {
+        return JSON.stringify(value, undefined, 4);
+    }
 }
 declare var __dirname: string; // Node-specific
 var global: NodeJS.Global = <any>Function("return this").call(undefined);