I have a custom add-in I wrote that I want loaded up when another excel file
is loaded. My code:

Dim o As Excel.AddIn

For Each o In Application.AddIns
If o.Name = "Planner.xla" Then
o.Installed = true
Exit For
End If
Next

However, when it tries to set the Installed property, it raises an error:

Runtime error 1004: Unable to set the installed property of the add-in
class.

Any suggestions on what to look for?