diff --git a/contrib/libfido2/src/iso7816.c b/contrib/libfido2/src/iso7816.c
index a11aae3e99d1..a4902277c6d8 100644
--- a/contrib/libfido2/src/iso7816.c
+++ b/contrib/libfido2/src/iso7816.c
@@ -59,6 +59,6 @@ iso7816_ptr(const iso7816_apdu_t *apdu)
 size_t
 iso7816_len(const iso7816_apdu_t *apdu)
 {
-	return apdu->alloc_len - sizeof(apdu->alloc_len) -
-	    sizeof(apdu->payload_len) - sizeof(apdu->payload_ptr);
+	return apdu->alloc_len - offsetof(iso7816_apdu_t, header) -
+	    (sizeof(iso7816_apdu_t) - offsetof(iso7816_apdu_t, payload));
 }
diff --git a/contrib/libfido2/src/iso7816.h b/contrib/libfido2/src/iso7816.h
index 5f5363a63a56..9bfad1fbab9d 100644
--- a/contrib/libfido2/src/iso7816.h
+++ b/contrib/libfido2/src/iso7816.h
@@ -27,14 +27,13 @@ struct iso7816_header {
 	uint8_t lc3;
 })
 
-PACKED_TYPE(iso7816_apdu_t,
-struct iso7816_apdu {
+typedef struct iso7816_apdu {
 	size_t            alloc_len;
 	uint16_t          payload_len;
 	uint8_t          *payload_ptr;
 	iso7816_header_t  header;
 	uint8_t           payload[];
-})
+} iso7816_apdu_t;
 
 const unsigned char *iso7816_ptr(const iso7816_apdu_t *);
 int iso7816_add(iso7816_apdu_t *, const void *, size_t);