.NET Workflow issue -

i calling wf:

        workflow1 wf = new workflow1()                            {                                childrenoption = true,                                market = "english",                                server = new uri("http://mysite"),                                sptags = datatoupdate,                            };         workflowinvoker.invoke(new workflow1()); 

but reason getting exception, why? other way how can invoke wf avoid problem?

values must bound non-null expression before parallelforeach activity  'parallelforeach<keyvaluepair<tuple<string,string,string>,zalupatag>>' can used. 

it appears workflow1 has parallelforeach activity in did not provide collection values property.