diff --git a/power-assert/power-assert-tests.ts b/power-assert/power-assert-tests.ts
index f7be7059f0..d21076b973 100644
--- a/power-assert/power-assert-tests.ts
+++ b/power-assert/power-assert-tests.ts
@@ -69,9 +69,9 @@ var customizedAssert2 = assert.customize({
         circular: '#@Circular#',
         lineSeparator: '\n',
         ambiguousEastAsianCharWidth: 2,
-        widthOf: () => null,
-        stringify: () => null,
-        diff: () => null,
+        widthOf: () => false,
+        stringify: () => false,
+        diff: () => false,
         writerClass: null,
         renderers: [
             './built-in/file',