diff --git a/NAMESPACE b/NAMESPACE
index dc0dded..ee502fe 100644
--- a/NAMESPACE
+++ b/NAMESPACE
@@ -46,7 +46,6 @@ export(repo_get)
 export(repo_resolve)
 export(repo_status)
 export(with_repo)
-if (getRversion() >= "4.0.0") importFrom(tools, R_user_dir)
 importFrom(R6,R6Class)
 importFrom(tools,file_ext)
 importFrom(utils,URLencode)
diff --git a/R/aaa-async.R b/R/aaa-async.R
index ac3da72..e5fc7d1 100644
--- a/R/aaa-async.R
+++ b/R/aaa-async.R
@@ -310,7 +310,7 @@ async_env <- new.env(parent = emptyenv())
 async_env$loops <- list()
 
 get_default_event_loop <- function() {
-  num_loops <- length(async_env$loops)
+  num_loops <- length2(async_env$loops)
   if (num_loops == 0) {
     err <- make_error(
       "You can only call async functions from an async context",
@@ -321,6 +321,10 @@ get_default_event_loop <- function() {
 
   async_env$loops[[num_loops]]
 }
+# Mockable version of length.
+length2 <- function(x) {
+  length(x)
+}
 
 push_event_loop <- function() {
   num_loops <- length(async_env$loops)
diff --git a/R/cache-dirs.R b/R/cache-dirs.R
index 9bca8dd..5b8880c 100644
--- a/R/cache-dirs.R
+++ b/R/cache-dirs.R
@@ -103,6 +103,8 @@ win_path_local <- function() {
 
 if (getRversion() < "4.0.0") {
   R_user_dir <- my_R_user_dir
+} else {
+  R_user_dir <- tools::R_user_dir
 }
 
 cleanup_old_cache_dir <- function(force = FALSE) {
diff --git a/R/mocks.R b/R/mocks.R
index 12e232a..332b2ce 100644
--- a/R/mocks.R
+++ b/R/mocks.R
@@ -1,11 +1,9 @@
 # Define these objects so they can be mocked in tests.
-interactive <- interactive
+interactive <- base::interactive
 readline <- function(prompt = "") {
-  readline
+  base::readline(prompt = prompt)
 }
-length <- length
 Sys.info <- function() {
-  Sys.info
+  base::Sys.info()
 }
-R_user_dir <- R_user_dir
-getRversion <- getRversion
+getRversion <- base::getRversion
diff --git a/tests/async/test-synchronise.R b/tests/async/test-synchronise.R
index 3453c4c..0a51e2f 100644
--- a/tests/async/test-synchronise.R
+++ b/tests/async/test-synchronise.R
@@ -1,7 +1,7 @@
 
 test_that("error if async function is called from sync context", {
 
-  local_mocked_bindings(length = function(...) 0)
+  local_mocked_bindings(length2 = function(x) 0)
   expect_error(
     get_default_event_loop(),
     class = "async_synchronization_barrier_error")