diff --git a/src/hsm/sc_hsm.c b/src/hsm/sc_hsm.c
index 69d7a3e..66daccb 100644
--- a/src/hsm/sc_hsm.c
+++ b/src/hsm/sc_hsm.c
@@ -513,9 +513,10 @@ int store_keys(void *key_ctx, int type, uint8_t key_id) {
     }
     else if (type & PICO_KEYS_KEY_EC) {
         mbedtls_ecdsa_context *ecdsa = (mbedtls_ecdsa_context *) key_ctx;
+        size_t olen = 0;
         kdata[0] = ecdsa->grp.id & 0xff;
-        mbedtls_ecp_write_key_ext(ecdsa, (size_t *)&key_size, kdata + 1, sizeof(kdata) - 1);
-        key_size++;
+        mbedtls_ecp_write_key_ext(ecdsa, &olen, kdata + 1, sizeof(kdata) - 1);
+        key_size = olen + 1;
     }
     else if (type & PICO_KEYS_KEY_AES) {
         if (type == PICO_KEYS_KEY_AES_128) {