diff --git a/src/openvpn/pkcs11_mbedtls.c b/src/openvpn/pkcs11_mbedtls.c
index bf9d953..2183217 100644
--- a/src/openvpn/pkcs11_mbedtls.c
+++ b/src/openvpn/pkcs11_mbedtls.c
@@ -64,7 +64,7 @@
 
     if (cert_blob_size > max_cert_size)
     {
-        msg(M_WARN, "PKCS#11: Certificate too large: %lu bytes, maximum is %lu", cert_blob_size, max_cert_size);
+        msg(M_WARN, "PKCS#11: Certificate too large: %zu bytes, maximum is %zu", cert_blob_size, max_cert_size);
         goto cleanup;
     }