error in DI for PCIe6535b using DAQ toolbox in matlab

Hello! I'm using PCIe 6535b for data acquisition in matlab with DAQ toolbox.

s = daq.createSession('ni')
addDigitalChannel(s,'dev1','Port0/Line0','InputOnly')

It worked fine with single acquisition:

inputSingleScan(s)

But when I try to add trigger:

addTriggerConnection(s,'External','Dev1/PFI5','StartTrigger')
dataIn = startForeground(s);

error happened:

To use the session object in clocked mode with the digital input channel 'port0/line0', an external clock must be supplied. See documentation on Digital Clocked Operations Using Session-Based Interface for more information.

I also tried to add clock:

addClockConnection(s,'external','Dev1/PFI5','ScanClock')
dataIn = startForeground(s);

error still happened:

NI Error -88700:
An internal error occurred.
 Task Name: _unnamedTask<1C>
Status Code: -88700

I want to realize continuous data acquisition with an external trigger. Could anyone give me some suggestion? Thanks a lot !