Fails test at calling `Docs.undocumented_names` in Julia stdlib

I’m editing Julia’s standard library and fail test at the following:

@testset "Docstrings" begin
    @test isempty(Docs.undocumented_names(Unicode))
Docstrings: Error During Test at /Users/paalon/git/julia/stdlib/Unicode/test/runtests.jl:550
  Test threw exception
  Expression: isempty(Docs.undocumented_names(Unicode))
  UndefVarError: `undocumented_names` not defined
   [1] getproperty(x::Module, f::Symbol)
     @ Base ./Base.jl:31
   [2] macro expansion
     @ ~/.julia/juliaup/ [inlined]
   [3] macro expansion
     @ ~/git/julia/stdlib/Unicode/test/runtests.jl:550 [inlined]
   [4] macro expansion
     @ ~/.julia/juliaup/ [inlined]
   [5] top-level scope
     @ ~/git/julia/stdlib/Unicode/test/runtests.jl:550
Test Summary: | Error  Total  Time
Docstrings    |     1      1  1.1s
ERROR: LoadError: Some tests did not pass: 0 passed, 0 failed, 1 errored, 0 broken.
in expression starting at /Users/paalon/git/julia/stdlib/Unicode/test/runtests.jl:549
ERROR: Package Unicode errored during testing

How to solve it?

Julia stdlib uses nightly functions so I need to test them in Julia nightly. There is Docs.undocumented_names in nightly.