Need to modify ComboBox RowSource from select statement to stored procedure

I have a legacy form function that populates a combo box using a hard-coded select statement.  It is noted below as Existing.

I want to simplify maintenance by referencing a stored procedure instead.  

My approach is not working and I would appreciate any pointers on the correct method and/or syntax.  The proposed code does not produce errors; the combo box is simply empty.  Please note that I need to keep the solution in this function.  Thanks!

''''''Existing''''''
 
Private Sub Form_Current()
On Error Resume Next
Me.Parent.Controls("txtActivityId") = [activity_id]
Me.Parent.Controls("subfrm_share_activity_steps").Form.Controls("step_key").RowSource = "select b.step_key, b.short_name, b.step_order from dbo.tbl_activity_step_type b where b.active = 1 and activity_key = " & Me.Controls("activity_key").Value & " order by step_order"
End Sub
 
''''''Proposed''''''
 
Private Sub Form_Current()
On Error Resume Next
Me.Parent.Controls("txtActivityId") = [activity_id]
Dim cmb As ComboBox
Dim sql As String
cmb = Me.Parent.Controls("subfrm_share_activity_steps").Form.Controls("step_key")
sql = "exec dbo.sp_get_activity_steps_available " & Me.Controls("activity_key").Value
cmb.RowSource = sql
End Sub

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