diff --git a/DESCRIPTION b/DESCRIPTION
index 241e1df..7db7197 100644
--- a/DESCRIPTION
+++ b/DESCRIPTION
@@ -1,48 +1,48 @@
 Package: wmf
 Type: Package
 Title: R Code for Wikimedia Foundation Internal Usage
-Version: 0.5.0
+Version: 0.5.1
 Date: 2018-07-05
 Authors@R: c(
     person("Mikhail", "Popov", email = "mikhail@wikimedia.org", role = c("aut", "cre")),
     person("Oliver", "Keyes", role = "aut", comment = "No longer employed at the Foundation"),
     person("Chelsy", "Xie", email = "cxie@wikimedia.org", role = "ctb")
     )
 Description: This package contains functions made for Analysts at Wikimedia
     Foundation, but can be used by people outside of the Foundation.
 License: MIT + file LICENSE
 URL: https://phabricator.wikimedia.org/diffusion/1821/
 BugReports: https://phabricator.wikimedia.org/maniphest/task/create/?
     projects=Product-Analytics
 Depends: R (>= 3.1.0)
 Imports:
     devtools,
     ggthemes (>= 3.4.0),
     ggplot2,
     jsonlite,
     lubridate,
     progress,
     purrr,
     pwr,
     Rcpp (>= 0.10.3),
     readr,
     RMySQL,
     tibble,
     urltools
 Suggests:
     lintr,
     testthat,
     knitr,
     rmarkdown,
     dplyr,
     tidyr
 LinkingTo:
     BH,
     Rcpp,
     RcppArmadillo (>= 0.3.810)
 NeedsCompilation: yes
 SystemRequirements: C++11
 LazyData: TRUE
 Roxygen: list(markdown = TRUE)
 RoxygenNote: 6.0.1
 VignetteBuilder: knitr
diff --git a/R/utils.R b/R/utils.R
index 2cff615..5128035 100644
--- a/R/utils.R
+++ b/R/utils.R
@@ -1,56 +1,55 @@
 #' @title Turn Null Into Character "NA"
 #' @description This function turns `NULL` in a list into character "NA", which
 #'   can be very useful when processing JSON data that has `null` values.
 #' @param x A list
 #' @return If any element from the input list is NULL, they will be turned into character
 #'  "NA". Otherwise, return the original list.
 #' @examples \dontrun{
 #' result <- mysql_read("SELECT userAgent FROM ...", "log")
 #' ua <- purrr::map_df(
 #'   result$userAgent,
 #'   ~ null2na(jsonlite::fromJSON(.x, simplifyVector = FALSE))
 #' )
 #' }
 #' @export
 null2na <- function(x) {
   return(lapply(x, function(y) {
     if (is.null(y)) {
       return(as.character(NA))
     } else {
       return(y)
     }
   }))
 }
 
 #' @title Parse character vector of JSON
 #' @description This is a shortcut for using [purrr::map()] with
 #'   [jsonlite::fromJSON()]. Useful when a column in a `data.frame` has JSON.
 #'   **Note**: if the intention is to have the parsed data in a column such as
 #'   when parsing in [dplyr::mutate()], it is *highly* recommended to switch
 #'   to a `tibble` via [tibble::as_tibble()] *first*.
 #' @param x A character vector of JSON
 #' @export
 parse_json <- function(x) {
   return(purrr::map(x, jsonlite::fromJSON))
 }
 
 #' @title Invert list
 #' @description Inverts a (named) list such that the values become the keys and
 #'   the keys become the values.
 #' @param x A named list
 #' @return A list with values as keys and keys as values.
 #' @examples
 #' invert_list(list(x = c(1, 2), y = c(2, 3)))
-#' # returns: list(`1` = "x", `2` = c("x", "y"), `3` = "y")
 #' @author Mikhail Popov
 #' @export
 invert_list <- function(x) {
   if (is.null(names(x))) stop("expecting input to be a named list")
   new_fields <- Reduce(union, x)
   if (length(new_fields) == 0) warning("inverted list will be empty")
   names(new_fields) <- new_fields
   return(lapply(new_fields, function(field) {
     y <- purrr::map_lgl(x, ~ field %in% .x)
     return(names(y)[y])
   }))
 }
diff --git a/man/invert_list.Rd b/man/invert_list.Rd
index 7e192bf..30fb719 100644
--- a/man/invert_list.Rd
+++ b/man/invert_list.Rd
@@ -1,25 +1,24 @@
 % Generated by roxygen2: do not edit by hand
 % Please edit documentation in R/utils.R
 \name{invert_list}
 \alias{invert_list}
 \title{Invert list}
 \usage{
 invert_list(x)
 }
 \arguments{
 \item{x}{A named list}
 }
 \value{
 A list with values as keys and keys as values.
 }
 \description{
 Inverts a (named) list such that the values become the keys and
 the keys become the values.
 }
 \examples{
 invert_list(list(x = c(1, 2), y = c(2, 3)))
-# returns: list(`1` = "x", `2` = c("x", "y"), `3` = "y")
 }
 \author{
 Mikhail Popov
 }
diff --git a/tests/testthat/test-utils.R b/tests/testthat/test-utils.R
index 019dba6..fd37c77 100644
--- a/tests/testthat/test-utils.R
+++ b/tests/testthat/test-utils.R
@@ -1,14 +1,22 @@
 context("Utilities")
 
 test_that("null2na", {
   # e.g. from jsonlite::fromJSON('{"a":1,"b":null,"c":"d"}'):
   x <- list(a = 1, b = NULL, c = "d")
   expect_equal(null2na(x), list(a = 1, b = as.character(NA), c = "d"))
 })
 
 test_that("parse_json", {
   x <- c('{"a":[1,2]}', '{"b":3,"c":[4,5]}')
   y <- parse_json(x)
   z <- list(list(a = c(1, 2)), list(b = 3, c = c(4, 5)))
   expect_equal(z, y)
 })
+
+test_that("invert_list", {
+  x <- list("x" = c(1, 2), y = c(), z = c(2, 3))
+  y <- list(`1` = "x", `2` = c("x", "z"), `3` = "z")
+  expect_equal(invert_list(x), y)
+  z <- suppressWarnings(invert_list(list(x = c(), y = c(), z = c())))
+  expect_equal(z, list())
+})