diff --git a/R/theme-current.R b/R/theme-current.R
index 396b53e95b..b56b2e1651 100644
--- a/R/theme-current.R
+++ b/R/theme-current.R
@@ -90,6 +90,7 @@ theme_get <- function() {
 #' @param new new theme (a list of theme elements)
 #' @export
 theme_set <- function(new) {
+  check_object(new, is.theme, "a {.cls theme} object")
   old <- ggplot_global$theme_current
   ggplot_global$theme_current <- new
   invisible(old)
diff --git a/tests/testthat/_snaps/theme.md b/tests/testthat/_snaps/theme.md
index 6fd9ed6eb3..0fe584926a 100644
--- a/tests/testthat/_snaps/theme.md
+++ b/tests/testthat/_snaps/theme.md
@@ -20,6 +20,10 @@
 
     Theme element `test` has "NULL" property without default: fill, colour, linewidth, and linetype
 
+---
+
+    `new` must be a <theme> object, not the string "foo".
+
 # element tree can be modified
 
     The `blablabla` theme element is not defined in the element hierarchy.
diff --git a/tests/testthat/test-theme.R b/tests/testthat/test-theme.R
index 3d64894b15..5ec9792457 100644
--- a/tests/testthat/test-theme.R
+++ b/tests/testthat/test-theme.R
@@ -264,6 +264,7 @@ test_that("incorrect theme specifications throw meaningful errors", {
   expect_snapshot_error(calc_element("line", theme(line = element_rect())))
   register_theme_elements(element_tree = list(test = el_def("element_rect")))
   expect_snapshot_error(calc_element("test", theme_gray() + theme(test = element_rect())))
+  expect_snapshot_error(theme_set("foo"))
 })
 
 test_that("element tree can be modified", {