diff --git a/.gitignore b/.gitignore
index 014c1b316..10ec9a9ad 100644
--- a/.gitignore
+++ b/.gitignore
@@ -7,3 +7,5 @@ docs
 .clangd
 launch.json
 .vscode/
+.cache/
+compile_commands.json
diff --git a/NEWS.md b/NEWS.md
index 2cb290516..fc859b73c 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -1,5 +1,8 @@
 # vctrs (development version)
 
+* `vec_detect_complete(NULL)` now returns `logical()`, consistent with
+  `vec_detect_missing(NULL)` (#1916).
+
 # vctrs 0.6.5
 
 * Internal changes requested by CRAN around C level format strings (#1896).
diff --git a/src/complete.c b/src/complete.c
index 1723f0c21..db7a47970 100644
--- a/src/complete.c
+++ b/src/complete.c
@@ -89,7 +89,8 @@ void vec_detect_complete_switch(SEXP x, R_len_t size, int* p_out) {
   case VCTRS_TYPE_raw: raw_detect_complete(x, size, p_out); break;
   case VCTRS_TYPE_list: list_detect_complete(x, size, p_out); break;
   case VCTRS_TYPE_dataframe: df_detect_complete(x, size, p_out); break;
-  case VCTRS_TYPE_scalar: r_stop_internal("Can't detect missing values in scalars.");
+  case VCTRS_TYPE_null: break;
+  case VCTRS_TYPE_scalar: stop_scalar_type(x, vec_args.empty, r_lazy_null);
   default: stop_unimplemented_vctrs_type("vec_detect_complete", vec_proxy_typeof(x));
   }
 }
@@ -169,12 +170,31 @@ void list_detect_complete(SEXP x, R_len_t size, int* p_out) {
 
 // -----------------------------------------------------------------------------
 
+static inline void col_detect_complete_switch(SEXP x, R_len_t size, int* p_out);
+
 static inline
 void df_detect_complete(SEXP x, R_len_t size, int* p_out) {
   r_ssize n_cols = r_length(x);
   const SEXP* p_x = VECTOR_PTR_RO(x);
 
   for (r_ssize i = 0; i < n_cols; ++i) {
-    vec_detect_complete_switch(p_x[i], size, p_out);
+    col_detect_complete_switch(p_x[i], size, p_out);
+  }
+}
+
+static inline
+void col_detect_complete_switch(SEXP x, R_len_t size, int* p_out) {
+  switch (vec_proxy_typeof(x)) {
+  case VCTRS_TYPE_logical: lgl_detect_complete(x, size, p_out); break;
+  case VCTRS_TYPE_integer: int_detect_complete(x, size, p_out); break;
+  case VCTRS_TYPE_double: dbl_detect_complete(x, size, p_out); break;
+  case VCTRS_TYPE_complex: cpl_detect_complete(x, size, p_out); break;
+  case VCTRS_TYPE_character: chr_detect_complete(x, size, p_out); break;
+  case VCTRS_TYPE_raw: raw_detect_complete(x, size, p_out); break;
+  case VCTRS_TYPE_list: list_detect_complete(x, size, p_out); break;
+  case VCTRS_TYPE_dataframe: r_stop_internal("Data frame columns should have been flattened by now.");
+  case VCTRS_TYPE_null: r_abort("Unexpected `NULL` column found in a data frame.");
+  case VCTRS_TYPE_scalar: stop_scalar_type(x, vec_args.empty, r_lazy_null);
+  default: stop_unimplemented_vctrs_type("vec_detect_complete", vec_proxy_typeof(x));
   }
 }
diff --git a/tests/testthat/_snaps/complete.md b/tests/testthat/_snaps/complete.md
new file mode 100644
index 000000000..954f505fa
--- /dev/null
+++ b/tests/testthat/_snaps/complete.md
@@ -0,0 +1,16 @@
+# catches `NULL` data frame columns
+
+    Code
+      vec_detect_complete(df)
+    Condition
+      Error in `vec_detect_complete()`:
+      ! Unexpected `NULL` column found in a data frame.
+
+# catches scalar objects
+
+    Code
+      vec_detect_complete(lm(1 ~ 1))
+    Condition
+      Error in `vec_size()`:
+      ! `x` must be a vector, not a <lm> object.
+
diff --git a/tests/testthat/_snaps/rank.md b/tests/testthat/_snaps/rank.md
index cc7ceef89..589704f58 100644
--- a/tests/testthat/_snaps/rank.md
+++ b/tests/testthat/_snaps/rank.md
@@ -1,3 +1,19 @@
+# `x` must not be `NULL` (#1823)
+
+    Code
+      vec_rank(NULL)
+    Condition
+      Error:
+      ! This type is not supported by `vec_order()`.
+
+---
+
+    Code
+      vec_rank(NULL, incomplete = "na")
+    Condition
+      Error:
+      ! This type is not supported by `vec_order()`.
+
 # `ties` is validated
 
     Code
diff --git a/tests/testthat/test-complete.R b/tests/testthat/test-complete.R
index 0ca0ca4a6..12f4ccfde 100644
--- a/tests/testthat/test-complete.R
+++ b/tests/testthat/test-complete.R
@@ -113,3 +113,22 @@ test_that("works with arrays", {
   expect_identical(vec_detect_complete(x), c(TRUE, FALSE))
   expect_identical(vec_detect_complete(y), c(TRUE, FALSE))
 })
+
+test_that("works with `NULL`", {
+  # Consistent with `vec_detect_missing()`
+  expect_identical(vec_detect_complete(NULL), logical())
+})
+
+test_that("catches `NULL` data frame columns", {
+  df <- new_data_frame(list(x = integer(), y = NULL), n = 0L)
+
+  expect_snapshot(error = TRUE, {
+    vec_detect_complete(df)
+  })
+})
+
+test_that("catches scalar objects", {
+  expect_snapshot(error = TRUE, {
+    vec_detect_complete(lm(1 ~ 1))
+  })
+})
diff --git a/tests/testthat/test-rank.R b/tests/testthat/test-rank.R
index 558173e8a..866d0625d 100644
--- a/tests/testthat/test-rank.R
+++ b/tests/testthat/test-rank.R
@@ -141,6 +141,15 @@ test_that("`x` must be a vector", {
   expect_error(vec_rank(identity), class = "vctrs_error_scalar_type")
 })
 
+test_that("`x` must not be `NULL` (#1823)", {
+  expect_snapshot(error = TRUE, {
+    vec_rank(NULL)
+  })
+  expect_snapshot(error = TRUE, {
+    vec_rank(NULL, incomplete = "na")
+  })
+})
+
 test_that("`ties` is validated", {
   expect_snapshot(error = TRUE, vec_rank(1, ties = "foo"))
   expect_snapshot(error = TRUE, vec_rank(1, ties = 1))