diff --git a/NEWS.md b/NEWS.md
index 6ae72571..4224459d 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -1,5 +1,9 @@
 # cpp11 (development version)
 
+* Removed implicit conversion operators from `sexp` to `double`, `bool`, and
+  `size_t`. Please use `cpp11::as_cpp()` for this instead, which is much safer
+  (#390).
+
 * `cpp11::writable::r_vector<T>::iterator` no longer implicitly deletes its
   copy assignment operator (#360).
 
diff --git a/cpp11test/src/test-function.cpp b/cpp11test/src/test-function.cpp
index 678a5ee2..22691a8c 100644
--- a/cpp11test/src/test-function.cpp
+++ b/cpp11test/src/test-function.cpp
@@ -5,22 +5,22 @@
 context("function-C++") {
   test_that("functions can be called") {
     auto median = cpp11::package("stats")["median"];
-    double res = median(cpp11::as_sexp({1., 2., 3., NA_REAL}), true);
-    expect_true(res == 2.);
+    cpp11::sexp res = median(cpp11::as_sexp({1., 2., 3., NA_REAL}), true);
+    expect_true(cpp11::as_cpp<double>(res) == 2.);
 
-    double res2 = median(cpp11::as_sexp({1., 2., 3., NA_REAL}), false);
-    expect_true(ISNAN(res2));
+    cpp11::sexp res2 = median(cpp11::as_sexp({1., 2., 3., NA_REAL}), false);
+    expect_true(ISNAN(cpp11::as_cpp<double>(res2)));
   }
 
   test_that("functions can be called with named arguments") {
     using namespace cpp11::literals;
 
     auto median = cpp11::package("stats")["median"];
-    double res = median("x"_nm = {1., 2., 3., NA_REAL}, "na.rm"_nm = true);
-    expect_true(res == 2.);
+    cpp11::sexp res = median("x"_nm = {1., 2., 3., NA_REAL}, "na.rm"_nm = true);
+    expect_true(cpp11::as_cpp<double>(res) == 2.);
 
-    double res2 = median(cpp11::as_sexp({1., 2., 3., NA_REAL}), false);
-    expect_true(ISNAN(res2));
+    cpp11::sexp res2 = median(cpp11::as_sexp({1., 2., 3., NA_REAL}), false);
+    expect_true(ISNAN(cpp11::as_cpp<double>(res2)));
   }
 
   test_that("base functions can be called") {
@@ -29,10 +29,9 @@ context("function-C++") {
     auto close = cpp11::package("base")["close"];
 
     cpp11::sexp con = file("foo");
+    cpp11::sexp res = isOpen(con);
 
-    bool res = isOpen(con);
-
-    expect_true(res == false);
+    expect_true(cpp11::as_cpp<bool>(res) == false);
 
     close(con);
   }
diff --git a/inst/include/cpp11/sexp.hpp b/inst/include/cpp11/sexp.hpp
index 7e9fdfea..25ced3c3 100644
--- a/inst/include/cpp11/sexp.hpp
+++ b/inst/include/cpp11/sexp.hpp
@@ -65,9 +65,7 @@ class sexp {
   }
 
   operator SEXP() const { return data_; }
-  operator double() const { return REAL_ELT(data_, 0); }
-  operator size_t() const { return REAL_ELT(data_, 0); }
-  operator bool() const { return LOGICAL_ELT(data_, 0); }
+
   SEXP data() const { return data_; }
 };