analogWrite for B4 and B5 not working?

Hello everybody,

I am trying to PWM the B4 and B5 pins using
analogWrite(B4, 0.5, {freq:10000});
but for some reason it just stays grounded. When I try the B3 pin it works fine. Is there a reason for this? I set the pinMode for B4 and B5 to 'af_output' which is what the B3 pin is set to, then I read it to make sure that they are all af_output using getPinMode(). Would this be because my espruino board is bad?

Thanks