diff --git a/pkcs11/yubihsm_pkcs11.c b/pkcs11/yubihsm_pkcs11.c
index f3129df0..d195cfe3 100644
--- a/pkcs11/yubihsm_pkcs11.c
+++ b/pkcs11/yubihsm_pkcs11.c
@@ -5667,7 +5667,7 @@ CK_DEFINE_FUNCTION(CK_RV, C_DeriveKey)
   ecdh_key.id = ECDH_KEY_TYPE << 16 | seq;
   ecdh_key.len = sizeof(ecdh_key.ecdh_key);
 
-  DBG_INFO("ecdh_key.id = %zu", ecdh_key.id);
+  DBG_INFO("ecdh_key.id = %lu", ecdh_key.id);
 
   if (value_len > ecdh_key.len) {
     DBG_ERR("Requested derived key is too long");