Adding a dimension with derived units in Unitful

New to Julia
1

Hello,
I am trying to add a dimension to Unitful. I can do that, and I can create (and use) a refunit, but I can’t seem to be able to create derived units.
In the code below, the commented line in the module (trying to create a bigthing) causes an error (ERROR: LoadError: ArgumentError: Symbol thing could not be found in unit modules Module[Unitful])

using Unitful
module MyUnits
    using Unitful
    @dimension Stuff "Stuff" TheStuff false
    @refunit thing "thing" thing Stuff true
    # @unit bigthing "bigthing" thebigthing 4u"thing" false
end
Unitful.register(MyUnits)
using .MyUnits
test = 1u"thing" |> u"kthing" # OK for predefined SI prefixes, but I would like a bigthing!

Could someone shed some light on what I am doing wrong?