mbedtls RSA trouble, mbedtls_rsa_import/mbedtls_rsa_complete fails


Working on RSA encryption, I'm trying to build an RSA context from just the P, Q and E part of the key (as per https://tls.mbed.org/kb/development/how-to-fill-rsa-context-from-n-e-p-and-q)

I have this sequence working on a windows machine, using the latest mbedtls release on visual studio 2019.

mbedtls_rsa_init(&ctx, MBEDTLS_RSA_PKCS_V15, 0);
ret = mbedtls_rsa_import(&ctx, NULL, &P, &Q, NULL, &E);
if (ret != 0)
    mbedtls_printf(" failed\n  ! mbedtls_rsa_import returned %d\n\n", ret);
if ((ret = mbedtls_rsa_complete(&ctx)) != 0)
     mbedtls_printf(" failed\n  ! mbedtls_rsa_complete returned %d\n\n", ret);

When doing the same thing on a nrf52840 board, the mbedtls_rsa_complete call fails with errorcode -16512, 'RSA - Bad input parameters to function'.

When I compare the RSA context structure after the mbedtls_rsa_import call, the contents are identical (nrf vs windows), using the same P/Q/E input arrays.

So, this might be a long shot here, but I'm not sure where to look now. The mbedtls code is rather hard to follow, sofar I have not been able to tell why the function fails. Both machines are little endian, so that shouldn't be it, right? I've tried setting a larger heap (CONFIG_MBEDTLS_HEAP_SIZE=2048), but that did not make any difference either.

Any pointers?