Unclear error message when extending a non-existing imported function

[KristofferC]

julia> module Foo
       import Base: f

       f(x) = x
       end
WARNING: Imported binding Base.f was undeclared at import time during import to Foo.
ERROR: invalid method definition in Foo: exported function Base.f does not exist

The error talks about "exported function Base.f" but f is not exported from Base.