can't run example of DigPulseTrain-Cont-Buff-ExtClk

Hi all,

I was trying to run NI DAQmx example of "DigPulseTrain-Cont-Buff-ExtClk", but there is an error generated once the external clock is supplied at frequency of 1 Hz as specified in the example. Error message as the following and I am using x-series PCI-e:

Sample Clock pulse occurred before a pulse could be generated using the previous pulse specification. Use a Sample Clock that is slower than the pulse train you want to generate.
Task Name: _unnamedTask<0>

Status Code: -201318

Any help would be highly appreciated.