From fa21441332588b2aa2ca343fa486d7435fc786cd Mon Sep 17 00:00:00 2001
From: "stainless-app[bot]"
 <142633134+stainless-app[bot]@users.noreply.github.com>
Date: Tue, 10 Feb 2026 20:31:14 +0000
Subject: [PATCH] fix(encoder): correctly serialize NullStruct

---
 packages/param/encoder.go      |  3 +++
 packages/param/encoder_test.go | 12 ++++++++++++
 2 files changed, 15 insertions(+)

diff --git a/packages/param/encoder.go b/packages/param/encoder.go
index 5b93734..c503c96 100644
--- a/packages/param/encoder.go
+++ b/packages/param/encoder.go
@@ -83,6 +83,9 @@ func MarshalUnion[T ParamStruct](metadata T, variants ...any) ([]byte, error) {
 		}
 	}
 	if nPresent == 0 || presentIdx == -1 {
+		if metadata.null() {
+			return []byte("null"), nil
+		}
 		if ovr, ok := metadata.Overrides(); ok {
 			return shimjson.Marshal(ovr)
 		}
diff --git a/packages/param/encoder_test.go b/packages/param/encoder_test.go
index d35914e..bf1b660 100644
--- a/packages/param/encoder_test.go
+++ b/packages/param/encoder_test.go
@@ -363,3 +363,15 @@ func TestOverriddenUnion(t *testing.T) {
 		})
 	}
 }
+
+func TestNullStructUnion(t *testing.T) {
+	nullUnion := param.NullStruct[PrimitiveUnion]()
+
+	b, err := json.Marshal(nullUnion)
+	if err != nil {
+		t.Fatalf("didn't expect error %v", err)
+	}
+	if string(b) != "null" {
+		t.Fatalf("expected null, received %s", string(b))
+	}
+}