diff --git a/NEWS.md b/NEWS.md
index 5b09a7c9..3f1607f8 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -8,6 +8,8 @@
 
 * `parameters()` and the `grid_*()` functions give more information in the error message when non-parameter objects are passed in (#437, #438).
 
+* `encode_unit()` now provides a helpful error when `x` is not a parameter object (#430).
+
 * `value_validate()`, `encode_unit()`, `get_p()`, `get_log_p()`, `get_n()`, `get_n_frac()`, `get_n_frac_range()`, and `get_batch_sizes()` now enforce empty dots (#439).
 
 
diff --git a/R/encode_unit.R b/R/encode_unit.R
index d4533fb2..58986ade 100644
--- a/R/encode_unit.R
+++ b/R/encode_unit.R
@@ -14,7 +14,6 @@
 #' @keywords internal
 #' @export
 encode_unit <- function(x, value, direction, ...) {
-  arg_match0(direction, values = c("forward", "backward"))
   UseMethod("encode_unit")
 }
 
@@ -29,6 +28,7 @@ encode_unit.default <- function(x, value, direction, ...) {
 #' @rdname encode_unit
 #' @export
 encode_unit.quant_param <- function(x, value, direction, original = TRUE, ...) {
+  arg_match0(direction, values = c("forward", "backward"))
   check_bool(original)
   if (has_unknowns(x)) {
     cli::cli_abort("The parameter object contains unknowns.")
@@ -70,6 +70,7 @@ encode_unit.quant_param <- function(x, value, direction, original = TRUE, ...) {
 #' @rdname encode_unit
 #' @export
 encode_unit.qual_param <- function(x, value, direction, ...) {
+  arg_match0(direction, values = c("forward", "backward"))
   check_dots_empty()
   if (has_unknowns(x)) {
     cli::cli_abort("The parameter object contains unknowns.")
diff --git a/tests/testthat/_snaps/encode_unit.md b/tests/testthat/_snaps/encode_unit.md
index 71689298..22bfa7ee 100644
--- a/tests/testthat/_snaps/encode_unit.md
+++ b/tests/testthat/_snaps/encode_unit.md
@@ -32,6 +32,14 @@
 
 # bad args
 
+    Code
+      encode_unit("foo")
+    Condition
+      Error in `encode_unit()`:
+      ! `x` should be a dials parameter object, not a string.
+
+---
+
     Code
       encode_unit(2, prune_method()$values, direction = "forward")
     Condition
diff --git a/tests/testthat/test-encode_unit.R b/tests/testthat/test-encode_unit.R
index 0777a385..e6bfeb94 100644
--- a/tests/testthat/test-encode_unit.R
+++ b/tests/testthat/test-encode_unit.R
@@ -71,6 +71,10 @@ test_that("bad args", {
   z_0 <- encode_unit(z, prune_method()$values, direction = "forward")
   x_0 <- encode_unit(x, 2:7, direction = "forward")
 
+  expect_snapshot(
+    error = TRUE,
+    encode_unit("foo")
+  )
   expect_snapshot(
     error = TRUE,
     encode_unit(2, prune_method()$values, direction = "forward")