diff --git a/R/helpers.R b/R/helpers.R
index 67963a6..5c0556f 100644
--- a/R/helpers.R
+++ b/R/helpers.R
@@ -19,7 +19,12 @@
 #'
 #' @export
 keep_units <- function(FUN, x, ..., unit=units(x)) {
-  set_units(do.call(FUN, list(x, ...)), unit, mode="standard")
+  if (inherits(try(unit, silent = TRUE), "symbolic_units")) {
+    set_units(do.call(FUN, list(x, ...)), unit, mode = "standard")
+  } else {
+    warning("`x` does not have units.")
+    do.call(FUN, list(x, ...))
+  }
 }
 
 dfapply <- function(X, FUN, ...) {
diff --git a/tests/testthat/test_helpers.R b/tests/testthat/test_helpers.R
index 37b1d33..4bfff58 100644
--- a/tests/testthat/test_helpers.R
+++ b/tests/testthat/test_helpers.R
@@ -3,3 +3,18 @@ test_that("keep_units restores units", {
 
   expect_identical(x, keep_units(drop_units, x))
 })
+
+test_that("keep_units warns when no units are provided", {
+  x <- 1:5
+
+  expect_warning(keep_units(sum, x))
+
+  expect_identical(suppressWarnings(keep_units(sum, x)), sum(x))
+})
+
+test_that("keep_units sets user-provided units", {
+  rate <- set_units(3, "1/min")
+  x <- keep_units(rexp, 3, rate, unit=units(1/rate))
+  expect_identical(units(x), units(1/rate))
+
+})