[mbed-tls] Signing without explicit private key

Hi all, I'm using mbedTLS libraries with an OPTIGA cryptochip. At the moment, when I call the sign function:

err = mbedtls_pk_sign(&priv_key, MBEDTLS_MD_SHA384, hash, 0, sign, &olen, mbedtls_ctr_drbg_random, &ctr_drbg);

I need to pass it a valid private key else if it isn't used, because alternative sign routine use the one into cryptochip.

It is possible to avoid passing this key?

Best regards, Stefano Mologni