diff --git a/engine/types/type42/linking_key.go b/engine/types/type42/linking_key.go
index ad8a37b30..060bceec8 100644
--- a/engine/types/type42/linking_key.go
+++ b/engine/types/type42/linking_key.go
@@ -24,8 +24,8 @@ func calculateHMAC(pubSharedSecret []byte, reference string) ([]byte, error) {
 	if reference == "" {
 		return nil, errors.New("invalid invoice number")
 	}
-	h := hmac.New(sha256.New, []byte(reference))
-	if _, err := h.Write(pubSharedSecret); err != nil {
+	h := hmac.New(sha256.New, pubSharedSecret)
+	if _, err := h.Write([]byte(reference)); err != nil {
 		return nil, fmt.Errorf("error writing HMAC message - %w", err)
 	}
 	return h.Sum(nil), nil
diff --git a/engine/types/type42/test_helpers.go b/engine/types/type42/test_helpers.go
index 1f1a098e6..b3f7e7183 100644
--- a/engine/types/type42/test_helpers.go
+++ b/engine/types/type42/test_helpers.go
@@ -22,7 +22,7 @@ func sha256Hash(data string) []byte {
 
 // Helper function to compute the expected HMAC
 func computeExpectedHMAC(pubSharedSecret []byte, reference string, idx int) []byte {
-	h := hmac.New(sha256.New, []byte(fmt.Sprintf("%s-%d", reference, idx)))
-	h.Write(pubSharedSecret)
+	h := hmac.New(sha256.New, pubSharedSecret)
+	h.Write([]byte(fmt.Sprintf("%s-%d", reference, idx)))
 	return h.Sum(nil)
 }