diff --git a/client.go b/client.go
index 9e32217..5bb1dc4 100644
--- a/client.go
+++ b/client.go
@@ -431,13 +431,17 @@ func (c *Client) RevokeLocalVaultInstallation(ctx context.Context, installationI
 
 func (c *Client) RelayLocalVaultEvent(ctx context.Context, installationID string, envelope M, payload M, envelopeHash, signatureHex, publicKeyHex string) (*EventReceipt, error) {
 	body := M{"envelope": envelope, "payload": payload}
+	raw, err := json.Marshal(body)
+	if err != nil {
+		return nil, fmt.Errorf("marshal local vault event: %w", err)
+	}
 	headers := map[string]string{
 		"X-Attesto-Local-Vault-Envelope-Hash": envelopeHash,
 		"X-Attesto-Local-Vault-Signature":     signatureHex,
 		"X-Attesto-Local-Vault-Public-Key":    publicKeyHex,
 	}
 	var out EventReceipt
-	err := c.requestRaw(ctx, http.MethodPost, "/v2/local-vault/installations/"+url.PathEscape(installationID)+"/events", nil, mustJSON(body), headers, "", &out)
+	err = c.requestRaw(ctx, http.MethodPost, "/v2/local-vault/installations/"+url.PathEscape(installationID)+"/events", nil, raw, headers, "", &out)
 	return &out, err
 }
 
@@ -650,14 +654,6 @@ func sleep(attempt int) {
 	time.Sleep(time.Duration(100*attempt) * time.Millisecond)
 }
 
-func mustJSON(value any) []byte {
-	raw, err := json.Marshal(value)
-	if err != nil {
-		panic(err)
-	}
-	return raw
-}
-
 func sanitizeMessage(message string) string {
 	for _, marker := range []string{"sk_live_", "sk_test_", "pk_live_", "pk_test_", "npm_", "pypi-", "atto_live_", "atto_test_"} {
 		if strings.Contains(message, marker) {
diff --git a/client_test.go b/client_test.go
index a8c2a1b..2427e20 100644
--- a/client_test.go
+++ b/client_test.go
@@ -105,3 +105,33 @@ func TestBearerClientCanCallTenantEndpoints(t *testing.T) {
 		t.Fatalf("unexpected streams: %#v", streams)
 	}
 }
+
+func TestRelayLocalVaultEventReturnsMarshalError(t *testing.T) {
+	called := false
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		called = true
+		t.Fatalf("request should not be sent when local vault event body cannot be marshaled")
+	}))
+	defer server.Close()
+
+	client, err := NewClient(testAPIKey, WithBaseURL(server.URL), WithMaxRetries(1))
+	if err != nil {
+		t.Fatalf("client: %v", err)
+	}
+
+	_, err = client.RelayLocalVaultEvent(
+		context.Background(),
+		"lv_123",
+		M{"source": "local-vault"},
+		M{"bad": func() {}},
+		strings.Repeat("a", 64),
+		strings.Repeat("b", 128),
+		strings.Repeat("c", 64),
+	)
+	if err == nil || !strings.Contains(err.Error(), "marshal local vault event") {
+		t.Fatalf("expected marshal error, got %v", err)
+	}
+	if called {
+		t.Fatalf("request was sent after marshal failure")
+	}
+}