diff --git a/src/openpgp/openpgp.c b/src/openpgp/openpgp.c
index 6ffec96..66f4e15 100644
--- a/src/openpgp/openpgp.c
+++ b/src/openpgp/openpgp.c
@@ -883,10 +883,12 @@ void make_rsa_response(mbedtls_rsa_context *rsa) {
 }
 
 void make_ecdsa_response(mbedtls_ecdsa_context *ecdsa) {
+    size_t plen = mbedtls_mpi_size(&ecdsa->grp.P);
     memcpy(res_APDU, "\x7f\x49\x81\x00", 4);
     res_APDU_size = 4;
     res_APDU[res_APDU_size++] = 0x86;
-    size_t plen = mbedtls_mpi_size(&ecdsa->grp.P);
+    res_APDU[res_APDU_size++] = 0x81;
+    res_APDU[res_APDU_size++] = 2*plen+1;
     res_APDU[res_APDU_size++] = 0x04;
     mbedtls_mpi_write_binary(&ecdsa->Q.X, res_APDU+res_APDU_size, plen); res_APDU_size += plen;
     mbedtls_mpi_write_binary(&ecdsa->Q.Y, res_APDU+res_APDU_size, plen); res_APDU_size += plen;