how did I acquire the position of particles in particle tracing module

I have a confusion which is to get the data of particles when I use matlab linking with comsol. But the function of mpheval is error.

For instance, qx=mpheval(model,{'qx'},'dataset','part1','t',0);

Unfortunately, there is

Error using mpheval (line 234)
Java exception occurred:
Exception:
com.comsol.nativejni.FlNativeException: Failed to evaluate expression
(rethrown as com.comsol.util.exceptions.FlException)
Messages:
Failed to evaluate expression

Undefined post expression


Have anybody help me, please.