diff --git a/packages/io-ts-http/src/combinators.ts b/packages/io-ts-http/src/combinators.ts
index 7f3f5f34..4c67aa28 100644
--- a/packages/io-ts-http/src/combinators.ts
+++ b/packages/io-ts-http/src/combinators.ts
@@ -43,6 +43,10 @@ const partialWithoutUndefined = <P extends t.Props>(
     },
     (a) => {
       const result = partialCodec.encode(a);
+      if (result === undefined) {
+        // `t.partial` will return this when passed `undefined` even though it is not in the type
+        return result;
+      }
       for (const key of Object.keys(result)) {
         if (result[key] === void 0) {
           delete result[key];
diff --git a/packages/io-ts-http/test/combinators.test.ts b/packages/io-ts-http/test/combinators.test.ts
index 295bb4ad..af67f106 100644
--- a/packages/io-ts-http/test/combinators.test.ts
+++ b/packages/io-ts-http/test/combinators.test.ts
@@ -113,6 +113,12 @@ describe('optionalized', () => {
     assertEncodes(optionalCodec, { a: undefined, b: 'foo' }, expected);
   });
 
+  it('returns undefined when encoding undefined', () => {
+    const optionalCodec = c.optionalized({});
+    const expected = undefined;
+    assertEncodes(optionalCodec, undefined, expected);
+  });
+
   it('decodes explicit null properties', () => {
     const nullCodec = c.optionalized({
       a: t.null,