Max/MSP crash with I/O vector size lower than 256

Dear all,
when I set the I/O vector size to a number lower than 256 (e.g. 128 or 64) and I turn on the DSP, even without any patch running any audio signal processing, I get that Max/MSP crashes (or better, it becomes unresponsive with the typical mac colored wheel running in place of the mouse pointer).

I am using Max/MSP 6.0.8 on a MacBook Pro OSX 10.9.2, Processor 2.7 GHz Intel Core i7.

I suspect that this problem might depend on the driver/soundcard. I use the standard CoreAudio driver, with input device the built-in microphone and output device the built-in output (sample rate = 44100)

I need to set the I/O vector size to 64, any suggestion?

Thanks in advance