diff --git a/src/utils.c b/src/utils.c
index e8b0810fd..72458923f 100644
--- a/src/utils.c
+++ b/src/utils.c
@@ -389,7 +389,9 @@ SEXP df_map(SEXP df, SEXP (*fn)(SEXP)) {
                                                                \
   r_ssize copy_size = (size > x_size) ? x_size : size;         \
                                                                \
-  memcpy(p_out, p_x, copy_size * sizeof(CTYPE));               \
+  if (copy_size > 0) {                                         \
+    memcpy(p_out, p_x, copy_size * sizeof(CTYPE));             \
+  }                                                            \
                                                                \
   UNPROTECT(1);                                                \
   return out;                                                  \
diff --git a/tests/testthat/test-set.R b/tests/testthat/test-set.R
index 7eb47287e..ce96b1cac 100644
--- a/tests/testthat/test-set.R
+++ b/tests/testthat/test-set.R
@@ -244,6 +244,11 @@ test_that("works with rcrds", {
   expect_identical(vec_set_union(y, x), vec_c(vec_slice(y, c(1, 2, 3, 4)), vec_slice(x, 1)))
 })
 
+test_that("works on empty inputs", {
+  expect_identical(vec_set_union(integer(), 12L), 12L)
+  expect_identical(vec_set_union(12L, integer()), 12L)
+})
+
 # vec_set_symmetric_difference --------------------------------------------
 
 test_that("retains names of `x` and `y` elements", {