How can I open the null input in SoX

Question

In libsox, I know the null output can be opened like so:

assert(out_r= sox_open_write("", &in_r->signal, NULL, "null", NULL, NULL));

But I'm not sure how to do this with the input (using sox_open_read). I'm using this to generate a sine wave using synth so right now, I have a workaround where I open a silent wave file, but this is far from elegant:

assert(in_tmp = sox_open_read("silence.wav", NULL, NULL, NULL));

Any ideas?