diff --git a/NAMESPACE b/NAMESPACE
index a9324c67..9e6981d9 100644
--- a/NAMESPACE
+++ b/NAMESPACE
@@ -17,12 +17,15 @@ S3method(grid_latin_hypercube,parameters)
 S3method(grid_max_entropy,list)
 S3method(grid_max_entropy,param)
 S3method(grid_max_entropy,parameters)
+S3method(grid_random,default)
 S3method(grid_random,list)
 S3method(grid_random,param)
 S3method(grid_random,parameters)
+S3method(grid_regular,default)
 S3method(grid_regular,list)
 S3method(grid_regular,param)
 S3method(grid_regular,parameters)
+S3method(grid_space_filling,default)
 S3method(grid_space_filling,list)
 S3method(grid_space_filling,param)
 S3method(grid_space_filling,parameters)
diff --git a/NEWS.md b/NEWS.md
index 06aacf69..aaee8beb 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -6,7 +6,7 @@
 
 * Parameters were added for the `tab_pfn` model: `num_estimators()`, `softmax_temperature()`, `balance_probabilities()`, `average_before_softmax()`, and `training_set_limit()`.
 
-* `parameters()` and the `grid_*()` functions give more information in the error message when non-parameter objects are passed in (#437).
+* `parameters()` and the `grid_*()` functions give more information in the error message when non-parameter objects are passed in (#437, #438).
 
 
 # dials 1.4.2
diff --git a/R/grids.R b/R/grids.R
index 052a9c30..477262ea 100644
--- a/R/grids.R
+++ b/R/grids.R
@@ -59,6 +59,24 @@ grid_regular <- function(x, ..., levels = 3, original = TRUE, filter = NULL) {
   UseMethod("grid_regular")
 }
 
+#' @export
+#' @rdname grid_regular
+grid_regular.default <- function(
+  x,
+  ...,
+  levels = 3,
+  original = TRUE,
+  filter = NULL
+) {
+  if (missing(x)) {
+    cli::cli_abort("At least one parameter object is required.")
+  }
+  cli::cli_abort(
+    "{.arg x} must be a {.cls param} object, list, or {.cls parameters} object, 
+    not {.obj_type_friendly {x}}."
+  )
+}
+
 #' @export
 #' @rdname grid_regular
 grid_regular.parameters <- function(
@@ -239,6 +257,24 @@ grid_random <- function(x, ..., size = 5, original = TRUE, filter = NULL) {
   UseMethod("grid_random")
 }
 
+#' @export
+#' @rdname grid_regular
+grid_random.default <- function(
+  x,
+  ...,
+  size = 5,
+  original = TRUE,
+  filter = NULL
+) {
+  if (missing(x)) {
+    cli::cli_abort("At least one parameter object is required.")
+  }
+  cli::cli_abort(
+    "{.arg x} must be a {.cls param} object, list, or {.cls parameters} object,
+    not {.obj_type_friendly {x}}."
+  )
+}
+
 #' @export
 #' @rdname grid_regular
 grid_random.parameters <- function(
diff --git a/R/space_filling.R b/R/space_filling.R
index e5f4a655..badfd5f3 100644
--- a/R/space_filling.R
+++ b/R/space_filling.R
@@ -107,6 +107,24 @@ grid_space_filling <- function(
   UseMethod("grid_space_filling")
 }
 
+#' @export
+#' @rdname grid_space_filling
+grid_space_filling.default <- function(
+  x,
+  ...,
+  size = 5,
+  type = "any",
+  original = TRUE
+) {
+  if (missing(x)) {
+    cli::cli_abort("At least one parameter object is required.")
+  }
+  cli::cli_abort(
+    "{.arg x} must be a {.cls param} object, list, or {.cls parameters} object,
+    not {.obj_type_friendly {x}}."
+  )
+}
+
 #' @export
 #' @rdname grid_space_filling
 grid_space_filling.parameters <- function(
diff --git a/man/grid_regular.Rd b/man/grid_regular.Rd
index 97234b4f..054cffc1 100644
--- a/man/grid_regular.Rd
+++ b/man/grid_regular.Rd
@@ -2,10 +2,12 @@
 % Please edit documentation in R/grids.R
 \name{grid_regular}
 \alias{grid_regular}
+\alias{grid_regular.default}
 \alias{grid_regular.parameters}
 \alias{grid_regular.list}
 \alias{grid_regular.param}
 \alias{grid_random}
+\alias{grid_random.default}
 \alias{grid_random.parameters}
 \alias{grid_random.list}
 \alias{grid_random.param}
@@ -13,6 +15,8 @@
 \usage{
 grid_regular(x, ..., levels = 3, original = TRUE, filter = NULL)
 
+\method{grid_regular}{default}(x, ..., levels = 3, original = TRUE, filter = NULL)
+
 \method{grid_regular}{parameters}(x, ..., levels = 3, original = TRUE, filter = NULL)
 
 \method{grid_regular}{list}(x, ..., levels = 3, original = TRUE, filter = NULL)
@@ -21,6 +25,8 @@ grid_regular(x, ..., levels = 3, original = TRUE, filter = NULL)
 
 grid_random(x, ..., size = 5, original = TRUE, filter = NULL)
 
+\method{grid_random}{default}(x, ..., size = 5, original = TRUE, filter = NULL)
+
 \method{grid_random}{parameters}(x, ..., size = 5, original = TRUE, filter = NULL)
 
 \method{grid_random}{list}(x, ..., size = 5, original = TRUE, filter = NULL)
diff --git a/man/grid_space_filling.Rd b/man/grid_space_filling.Rd
index 7571d6d1..b242cd7f 100644
--- a/man/grid_space_filling.Rd
+++ b/man/grid_space_filling.Rd
@@ -2,6 +2,7 @@
 % Please edit documentation in R/space_filling.R
 \name{grid_space_filling}
 \alias{grid_space_filling}
+\alias{grid_space_filling.default}
 \alias{grid_space_filling.parameters}
 \alias{grid_space_filling.list}
 \alias{grid_space_filling.param}
@@ -9,6 +10,8 @@
 \usage{
 grid_space_filling(x, ..., size = 5, type = "any", original = TRUE)
 
+\method{grid_space_filling}{default}(x, ..., size = 5, type = "any", original = TRUE)
+
 \method{grid_space_filling}{parameters}(
   x,
   ...,
diff --git a/tests/testthat/_snaps/grids.md b/tests/testthat/_snaps/grids.md
index 9ebb1360..570203b8 100644
--- a/tests/testthat/_snaps/grids.md
+++ b/tests/testthat/_snaps/grids.md
@@ -96,8 +96,16 @@
     Code
       grid_random()
     Condition
-      Error in `UseMethod()`:
-      ! no applicable method for 'grid_random' applied to an object of class "NULL"
+      Error in `grid_random()`:
+      ! At least one parameter object is required.
+
+---
+
+    Code
+      grid_random("not a param")
+    Condition
+      Error in `grid_random()`:
+      ! `x` must be a <param> object, list, or <parameters> object, not a string.
 
 ---
 
@@ -250,8 +258,16 @@
     Code
       grid_regular()
     Condition
-      Error in `UseMethod()`:
-      ! no applicable method for 'grid_regular' applied to an object of class "NULL"
+      Error in `grid_regular()`:
+      ! At least one parameter object is required.
+
+---
+
+    Code
+      grid_regular("not a param")
+    Condition
+      Error in `grid_regular()`:
+      ! `x` must be a <param> object, list, or <parameters> object, not a string.
 
 ---
 
diff --git a/tests/testthat/_snaps/space_filling.md b/tests/testthat/_snaps/space_filling.md
index dca7a9ab..c97ff793 100644
--- a/tests/testthat/_snaps/space_filling.md
+++ b/tests/testthat/_snaps/space_filling.md
@@ -118,8 +118,16 @@
     Code
       grid_space_filling()
     Condition
-      Error in `UseMethod()`:
-      ! no applicable method for 'grid_space_filling' applied to an object of class "NULL"
+      Error in `grid_space_filling()`:
+      ! At least one parameter object is required.
+
+---
+
+    Code
+      grid_space_filling("not a param")
+    Condition
+      Error in `grid_space_filling()`:
+      ! `x` must be a <param> object, list, or <parameters> object, not a string.
 
 ---
 
diff --git a/tests/testthat/test-grids.R b/tests/testthat/test-grids.R
index 743a42f8..bfde256a 100644
--- a/tests/testthat/test-grids.R
+++ b/tests/testthat/test-grids.R
@@ -109,8 +109,9 @@ test_that("grid_random validates inputs", {
 })
 
 test_that("grid_random() errors with non-param inputs", {
-  # no input at all
+  # default method
   expect_snapshot(error = TRUE, grid_random())
+  expect_snapshot(error = TRUE, grid_random("not a param"))
 
   # param method
   expect_snapshot(error = TRUE, grid_random(penalty(), "min_n"))
@@ -157,8 +158,9 @@ test_that("grid_regular validates inputs", {
 })
 
 test_that("grid_regular() errors with non-param inputs", {
-  # no input at all
+  # default method
   expect_snapshot(error = TRUE, grid_regular())
+  expect_snapshot(error = TRUE, grid_regular("not a param"))
 
   # param method
   expect_snapshot(error = TRUE, grid_regular(penalty(), "min_n"))
diff --git a/tests/testthat/test-space_filling.R b/tests/testthat/test-space_filling.R
index c13b6efb..4b2d1146 100644
--- a/tests/testthat/test-space_filling.R
+++ b/tests/testthat/test-space_filling.R
@@ -343,8 +343,9 @@ test_that("grid_space_filling validates inputs", {
 })
 
 test_that("grid_space_filling() errors with non-param inputs", {
-  # no input at all
+  # default method
   expect_snapshot(error = TRUE, grid_space_filling())
+  expect_snapshot(error = TRUE, grid_space_filling("not a param"))
 
   # param method
   expect_snapshot(error = TRUE, grid_space_filling(penalty(), "min_n"))