ExUnit.Parameterized causes "warning: variable does not exist" (but runs fine?)

I am trying to use ExUnit.Parameterized. If I try to run the exact example module in the documentation, then I get a warning on this line:

  # support Callback as `context`
  test_with_params "add params with context", context,
    fn (a, b, expected) ->
      assert a + b == expected
    end do
        {context[:value], 2, 3}  # *** warning here

The warning is warning: variable "context" does not exist and is being expanded to "context()", please use parentheses to remove the ambiguity or change the variable name

Interestingly, if I disable warnings as errors the test runs and passes just fine.