diff --git a/lib/internal/util/comparisons.js b/lib/internal/util/comparisons.js
index 5b29ccf561c..0e58ea2cb4e 100644
--- a/lib/internal/util/comparisons.js
+++ b/lib/internal/util/comparisons.js
@@ -120,6 +120,9 @@ function strictDeepEqual(val1, val2, memos) {
     const val1Value = val1.valueOf();
     // Note: Boxed string keys are going to be compared again by Object.keys
     if (val1Value !== val1) {
+      if (typeof val2.valueOf !== 'function') {
+        return false;
+      }
       if (!innerDeepEqual(val1Value, val2.valueOf(), true))
         return false;
       // Fast path for boxed primitives
diff --git a/test/parallel/test-assert-deep.js b/test/parallel/test-assert-deep.js
index b335f95a511..08491421f62 100644
--- a/test/parallel/test-assert-deep.js
+++ b/test/parallel/test-assert-deep.js
@@ -890,3 +890,10 @@ assert.deepStrictEqual(obj1, obj2);
   );
   util.inspect.defaultOptions = tmp;
 }
+
+// Basic valueOf check.
+{
+  const a = new String(1);
+  a.valueOf = undefined;
+  assertNotDeepOrStrict(a, new String(1));
+}