diff --git a/src/index.test.ts b/src/index.test.ts
index 249bbb0..356ab85 100644
--- a/src/index.test.ts
+++ b/src/index.test.ts
@@ -197,6 +197,25 @@ describe('stringify & parse', () => {
       },
     },
 
+    'works for invalid dates': {
+      input: {
+        date: new Date('invalid'),
+      },
+      output: {
+        date: '',
+      },
+      outputAnnotations: {
+        values: {
+          date: ['Date'],
+        },
+      },
+      dontExpectEquality: true,
+      customExpectations: value => {
+        expect(value.date).toBeInstanceOf(Date);
+        expect(Number.isNaN(value.date.getTime())).toBe(true);
+      },
+    },
+
     'works for Errors': {
       input: {
         e: new Error('epic fail'),
diff --git a/src/transformer.ts b/src/transformer.ts
index cce7d55..536a07a 100644
--- a/src/transformer.ts
+++ b/src/transformer.ts
@@ -1,6 +1,5 @@
 import {
   isBigint,
-  isDate,
   isInfinite,
   isMap,
   isNaNValue,
@@ -73,9 +72,12 @@ const simpleRules = [
     }
   ),
   simpleTransformation(
-    isDate,
+    (v): v is Date => v instanceof Date,
     'Date',
-    v => v.toISOString(),
+    // An invalid Date (`new Date(NaN)`) has no ISO representation and would
+    // otherwise be dropped to `null`, losing its type. Serialize it to an
+    // empty string, which `new Date('')` revives back into an invalid Date.
+    v => (isNaN(v.valueOf()) ? '' : v.toISOString()),
     v => new Date(v)
   ),