I am not an expert at creating sub-reports however in messing around with this on my machine I found:

This may be because the parameter you are supplying is a Query Parameter instead of a Report Parameter. I was able to pass a work item ID by doing the following:

On your sub-report change your Report Parameter to be Non-queried.
From the Data or Layout Tab: Report -> Report Parameters
Change Available Values: "From query" to "Non-queried" - May also want to change from Mulit-Value to Single-Value if you are displaying a single work items properties

After these changes my Report Parameter for my subreport was as follows:
Name: WorkItemSystemId
Data type: String
Hidden checked, all other not checked
Available values: "Non-queried", no values added.
Default values: null

On the report including the sub-report:
On Layout tab: Right click the subreport -> Properties
General tab: Subreport: <Sub report name>
Parameters tab: Parameter Name: WorkItemSystemId
Parameter Value: [Work Item].[System_Id].&[2]
OK
In the Properties window (not the one you had open , then one under View -> Properties Window) change the NoRows property to somthing like "This subreport contains no data" so something is displayed when your query in your subreport doesn't return any data.

Save, Preview ... you should see your data.

Depending on your query in your subreport you may need to edit the Parameter Value.