Timing synchronize options between PXI E series and X series module

Hi,

I have a PXI-4496 and PXI-6723 card attached into the PXI 1033-chassis, and I was attempting to synchronize the timing between these two card through sample clock synchronization (PXI-6723 as master, PXI-4496 as slave). However, I have encountered an error -200077 saying that the only possible value to set the timing source for the PXI-4496 input task is either an empty string or an onboard clock, and the PXI1Slot6/ao/sampleClock is not a supported value. 

May I know how am I supposed to sync these two cards together?