Requery subform Field based on another Subform Field's selection

[Shanty]

I need some help trying to figure out something in Access 2003. I have a
main form 'A' with a subform 'B' linked using the EmployeeID=Employee_ID
fields of A and B respectively. The subform loads the filtered data based on
the employeeID in the subform just fine but what I'm trying to do is add a
record to the subform for the employee regarding a course request.

In the subform (form B) I have a combobox whose RecordSource is the
VendorName from the Vendor table and the ControlSource is a field in another
table called tblCrseReq. Upon selecting the vendor, the VendorName for
"CoyName" field on the subform is used to filter the available courses from
that vendor int he "Course_ID" field of that same form. This works fine
using the subform's code (below) when I run the form outside of the main form
however, it doesn't work within the main form as a subform. It prompts me
for the value of the CoyName field.
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Private Sub CoyName_AfterUpdate()

Me.Course_ID = Null
Me.Course_ID.Requery
Me.Course_ID = Me.Course_ID.ItemData(0)

End Sub