diff --git a/NEWS.md b/NEWS.md
index 9e115794b..41736454f 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -13,13 +13,13 @@
 
 * The xgboost engine for boosted trees was translating `mtry` to xgboost's `colsample_bytree`. We now map `mtry` to `colsample_bynode` since that is more consistent with how random forest works. `colsample_bytree` can still be optimized by passing it in as an engine argument. `colsample_bynode` was added to xgboost after the `parsnip` package code was written. (#495)
 
-* For xgboost boosting, `mtry` and `colsample_bytree` can be passed as integer counts or proportions while `subsample` and `validation` should be proportions. `xgb_train()` now has a new option `counts` for state what scale `mtry` and `colsample_bytree` are being used. (#461)  
+* For xgboost, `mtry` and `colsample_bytree` can be passed as integer counts or proportions, while `subsample` and `validation` should always be proportions. `xgb_train()` now has a new option `counts` (`TRUE` or `FALSE`) that states which scale for `mtry` and `colsample_bytree` is being used. (#461)  
 
 ## Other Changes
 
 * Re-licensed package from GPL-2 to MIT. See [consent from copyright holders here](https://github.com/tidymodels/parsnip/issues/462).
 
-* `set_mode()` now checks if `mode` is compatible with the model class, similar to `new_model_spec()` (@jtlandis, #467).
+* `set_mode()` now checks if `mode` is compatible with the model class, similar to `new_model_spec()` (@jtlandis, #467). Both `set_mode()` and `set_engine()` now error for `NULL` or missing arguments (#503).
 
 * Re-organized model documentation for `update` methods (#479).
 
diff --git a/R/aaa_models.R b/R/aaa_models.R
index 2a40e3d11..f7f1b3824 100644
--- a/R/aaa_models.R
+++ b/R/aaa_models.R
@@ -133,21 +133,23 @@ check_mode_val <- function(mode) {
   invisible(NULL)
 }
 
+
+stop_incompatible_mode <- function(spec_modes) {
+  msg <- glue::glue(
+    "Available modes are: ",
+    glue::glue_collapse(glue::glue("'{spec_modes}'"), sep = ", ")
+  )
+  rlang::abort(msg)
+}
+
 # check if class and mode are compatible
 check_spec_mode_val <- function(cls, mode) {
   spec_modes <- rlang::env_get(get_model_env(), paste0(cls, "_modes"))
-  compatible_modes <-
-    glue::glue(
-      "`mode` should be one of: ",
-      glue::glue_collapse(glue::glue("'{spec_modes}'"), sep = ", ")
-    )
-
-  if (is.null(mode)) {
-    rlang::abort(compatible_modes)
+  if (is.null(mode) || length(mode) > 1) {
+    stop_incompatible_mode(spec_modes)
   } else if (!(mode %in% spec_modes)) {
-    rlang::abort(compatible_modes)
+    stop_incompatible_mode(spec_modes)
   }
-
   invisible(NULL)
 }
 
diff --git a/R/arguments.R b/R/arguments.R
index ed2b995a7..8c7cb735b 100644
--- a/R/arguments.R
+++ b/R/arguments.R
@@ -76,11 +76,12 @@ set_args <- function(object, ...) {
 #' @rdname set_args
 #' @export
 set_mode <- function(object, mode) {
+  cls <- class(object)[1]
   if (rlang::is_missing(mode)) {
-    mode <- NULL
+    spec_modes <- rlang::env_get(get_model_env(), paste0(cls, "_modes"))
+    stop_incompatible_mode(spec_modes)
   }
-  mode <- mode[1]
-  check_spec_mode_val(class(object)[1], mode)
+  check_spec_mode_val(cls, mode)
   object$mode <- mode
   object
 }
diff --git a/R/engines.R b/R/engines.R
index 1babb0186..e6000c184 100644
--- a/R/engines.R
+++ b/R/engines.R
@@ -10,23 +10,21 @@ possible_engines <- function(object, ...) {
   unique(engs$engine)
 }
 
+stop_incompatible_engine <- function(avail_eng) {
+  msg <- glue::glue(
+    "Available engines are: ",
+    glue::glue_collapse(glue::glue("'{avail_eng}'"), sep = ", ")
+  )
+  rlang::abort(msg)
+}
+
 check_engine <- function(object) {
   avail_eng <- possible_engines(object)
-  if (is.null(object$engine)) {
-    object$engine <- avail_eng[1]
-    rlang::warn(glue::glue("`engine` was NULL and updated to be `{object$engine}`"))
-  } else {
-    if (!is.character(object$engine) | length(object$engine) != 1) {
-      rlang::abort("`engine` should be a single character value.")
-    }
-  }
-  if (!(object$engine %in% avail_eng)) {
-    rlang::abort(
-      glue::glue(
-        "Engine '{object$engine}' is not available. Please use one of: ",
-        glue::glue_collapse(glue::glue("'{avail_eng}'"), sep = ", ")
-      )
-    )
+  eng <- object$engine
+  if (is.null(eng) || length(eng) > 1) {
+    stop_incompatible_engine(avail_eng)
+  } else if (!(eng %in% avail_eng)) {
+    stop_incompatible_engine(avail_eng)
   }
   object
 }
@@ -97,7 +95,8 @@ set_engine <- function(object, engine, ...) {
   }
 
   if (rlang::is_missing(engine)) {
-    engine <- NULL
+    avail_eng <- possible_engines(object)
+    stop_incompatible_engine(avail_eng)
   }
   object$engine <- engine
   object <- check_engine(object)
diff --git a/tests/testthat/test_args_and_modes.R b/tests/testthat/test_args_and_modes.R
index 9abf5807e..be58c7933 100644
--- a/tests/testthat/test_args_and_modes.R
+++ b/tests/testthat/test_args_and_modes.R
@@ -49,6 +49,6 @@ test_that('pipe engine', {
 test_that("can't set a mode that isn't allowed by the model spec", {
   expect_error(
     set_mode(linear_reg(), "classification"),
-    "`mode` should be one of"
+    "Available modes are:"
   )
 })
diff --git a/tests/testthat/test_mars.R b/tests/testthat/test_mars.R
index ca5b3e650..43c795c78 100644
--- a/tests/testthat/test_mars.R
+++ b/tests/testthat/test_mars.R
@@ -111,7 +111,7 @@ test_that('updating', {
 })
 
 test_that('bad input', {
-  expect_warning(translate(mars(mode = "regression") %>% set_engine()))
+  expect_error(translate(mars(mode = "regression") %>% set_engine()))
   expect_error(translate(mars() %>% set_engine("wat?")))
   expect_error(translate(mars(formula = y ~ x)))
 })
diff --git a/tests/testthat/test_multinom_reg.R b/tests/testthat/test_multinom_reg.R
index 3bbcea84a..9a229f496 100644
--- a/tests/testthat/test_multinom_reg.R
+++ b/tests/testthat/test_multinom_reg.R
@@ -123,6 +123,6 @@ test_that('updating', {
 test_that('bad input', {
   expect_error(multinom_reg(mode = "regression"))
   expect_error(translate(multinom_reg(penalty = 0.1) %>% set_engine("wat?")))
-  expect_warning(multinom_reg(penalty = 0.1) %>% set_engine())
+  expect_error(multinom_reg(penalty = 0.1) %>% set_engine())
   expect_warning(translate(multinom_reg(penalty = 0.1) %>% set_engine("glmnet", x = hpc[,1:3], y = hpc$class)))
 })
diff --git a/tests/testthat/test_nearest_neighbor.R b/tests/testthat/test_nearest_neighbor.R
index a550fa4ba..2ae8a671c 100644
--- a/tests/testthat/test_nearest_neighbor.R
+++ b/tests/testthat/test_nearest_neighbor.R
@@ -122,5 +122,5 @@ test_that('updating', {
 
 test_that('bad input', {
   expect_error(nearest_neighbor(mode = "reallyunknown"))
-  expect_warning(nearest_neighbor() %>% set_engine( NULL))
+  expect_error(nearest_neighbor() %>% set_engine( NULL))
 })
diff --git a/tests/testthat/test_nullmodel.R b/tests/testthat/test_nullmodel.R
index 9a5af12bf..5590e2d5d 100644
--- a/tests/testthat/test_nullmodel.R
+++ b/tests/testthat/test_nullmodel.R
@@ -32,7 +32,7 @@ test_that('engine arguments', {
 })
 
 test_that('bad input', {
-  expect_warning(translate(null_model(mode = "regression") %>% set_engine()))
+  expect_error(translate(null_model(mode = "regression") %>% set_engine()))
   expect_error(translate(null_model() %>% set_engine("wat?")))
   expect_error(translate(null_model(formula = y ~ x)))
   expect_warning(
diff --git a/tests/testthat/test_rand_forest.R b/tests/testthat/test_rand_forest.R
index b8dbf361f..926a8b899 100644
--- a/tests/testthat/test_rand_forest.R
+++ b/tests/testthat/test_rand_forest.R
@@ -192,7 +192,7 @@ test_that('updating', {
 })
 
 test_that('bad input', {
-  expect_warning(translate(rand_forest(mode = "classification") %>% set_engine(NULL)))
+  expect_error(translate(rand_forest(mode = "classification") %>% set_engine(NULL)))
   expect_error(rand_forest(mode = "time series"))
   expect_error(translate(rand_forest(mode = "classification") %>% set_engine("wat?")))
   expect_error(translate(rand_forest(mode = "classification", ytest = 2)))
diff --git a/tests/testthat/test_surv_reg.R b/tests/testthat/test_surv_reg.R
index 1947bed41..424f48a9b 100644
--- a/tests/testthat/test_surv_reg.R
+++ b/tests/testthat/test_surv_reg.R
@@ -85,7 +85,7 @@ test_that('bad input', {
 
   expect_error(surv_reg(mode = ", classification"))
   expect_error(translate(surv_reg() %>% set_engine("wat")))
-  expect_warning(translate(surv_reg() %>% set_engine(NULL)))
+  expect_error(translate(surv_reg() %>% set_engine(NULL)))
 })
 
 test_that("deprecation warning", {
diff --git a/tests/testthat/test_svm_linear.R b/tests/testthat/test_svm_linear.R
index 82b3e7a42..e53540980 100644
--- a/tests/testthat/test_svm_linear.R
+++ b/tests/testthat/test_svm_linear.R
@@ -104,7 +104,7 @@ test_that('updating', {
 })
 
 test_that('bad input', {
-  expect_warning(translate(svm_linear(mode = "regression") %>% set_engine( NULL)))
+  expect_error(translate(svm_linear(mode = "regression") %>% set_engine( NULL)))
   expect_error(svm_linear(mode = "reallyunknown"))
   expect_error(translate(svm_linear(mode = "regression") %>% set_engine("LiblineaR", type = 3)))
   expect_error(translate(svm_linear(mode = "classification") %>% set_engine("LiblineaR", type = 11)))
diff --git a/tests/testthat/test_svm_liquidsvm.R b/tests/testthat/test_svm_liquidsvm.R
index 2a966b8e9..cef99ec9d 100644
--- a/tests/testthat/test_svm_liquidsvm.R
+++ b/tests/testthat/test_svm_liquidsvm.R
@@ -77,5 +77,5 @@ test_that('updating', {
 
 test_that('bad input', {
   expect_error(svm_rbf(mode = "reallyunknown"))
-  expect_warning(svm_rbf() %>% set_engine( NULL))
+  expect_error(svm_rbf() %>% set_engine( NULL))
 })
diff --git a/tests/testthat/test_svm_poly.R b/tests/testthat/test_svm_poly.R
index fd403560f..bea7d5093 100644
--- a/tests/testthat/test_svm_poly.R
+++ b/tests/testthat/test_svm_poly.R
@@ -106,7 +106,7 @@ test_that('updating', {
 
 test_that('bad input', {
   expect_error(svm_poly(mode = "reallyunknown"))
-  expect_warning(svm_poly() %>% set_engine(NULL))
+  expect_error(svm_poly() %>% set_engine(NULL))
 })
 
 # ------------------------------------------------------------------------------
diff --git a/tests/testthat/test_svm_rbf.R b/tests/testthat/test_svm_rbf.R
index 9541f1792..059754fbf 100644
--- a/tests/testthat/test_svm_rbf.R
+++ b/tests/testthat/test_svm_rbf.R
@@ -87,7 +87,7 @@ test_that('updating', {
 
 test_that('bad input', {
   expect_error(svm_rbf(mode = "reallyunknown"))
-  expect_warning(translate(svm_rbf(mode = "regression") %>% set_engine( NULL)))
+  expect_error(translate(svm_rbf(mode = "regression") %>% set_engine( NULL)))
 })
 
 # ------------------------------------------------------------------------------