diff --git a/inst/include/cpp11/as.hpp b/inst/include/cpp11/as.hpp
index 682f12b5..d9a94692 100644
--- a/inst/include/cpp11/as.hpp
+++ b/inst/include/cpp11/as.hpp
@@ -92,22 +92,21 @@ template <typename T>
 enable_if_integral<T, T> as_cpp(SEXP from) {
   if (Rf_isInteger(from)) {
     if (Rf_xlength(from) == 1) {
-      return INTEGER_ELT(from, 0);
+      return static_cast<T>(INTEGER_ELT(from, 0));
     }
   } else if (Rf_isReal(from)) {
     if (Rf_xlength(from) == 1) {
-      if (ISNA(REAL_ELT(from, 0))) {
-        return NA_INTEGER;
-      }
       double value = REAL_ELT(from, 0);
-      if (is_convertible_without_loss_to_integer(value)) {
-        return value;
+      if (ISNA(value) && sizeof(T) >= sizeof(int)) {
+        return static_cast<T>(NA_INTEGER);
+      } else if (!ISNA(value) && is_convertible_without_loss_to_integer(value)) {
+        return static_cast<T>(value);
       }
     }
   } else if (Rf_isLogical(from)) {
     if (Rf_xlength(from) == 1) {
-      if (LOGICAL_ELT(from, 0) == NA_LOGICAL) {
-        return NA_INTEGER;
+      if (LOGICAL_ELT(from, 0) == NA_LOGICAL && sizeof(T) >= sizeof(int)) {
+        return static_cast<T>(NA_INTEGER);
       }
     }
   }
diff --git a/inst/include/cpp11/r_string.hpp b/inst/include/cpp11/r_string.hpp
index 692b66ea..edef4431 100644
--- a/inst/include/cpp11/r_string.hpp
+++ b/inst/include/cpp11/r_string.hpp
@@ -17,7 +17,7 @@ class r_string {
   r_string(SEXP data) : data_(data) {}
   r_string(const char* data) : data_(safe[Rf_mkCharCE](data, CE_UTF8)) {}
   r_string(const std::string& data)
-      : data_(safe[Rf_mkCharLenCE](data.c_str(), data.size(), CE_UTF8)) {}
+      : data_(safe[Rf_mkCharLenCE](data.c_str(), static_cast<int>(data.size()), CE_UTF8)) {}
 
   operator SEXP() const { return data_; }
   operator sexp() const { return data_; }
diff --git a/inst/include/cpp11/sexp.hpp b/inst/include/cpp11/sexp.hpp
index 9f6f5e18..eef0cf49 100644
--- a/inst/include/cpp11/sexp.hpp
+++ b/inst/include/cpp11/sexp.hpp
@@ -76,7 +76,7 @@ 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 size_t() const { return static_cast<size_t>(REAL_ELT(data_, 0)); }
   operator bool() const { return LOGICAL_ELT(data_, 0); }
   SEXP data() const { return data_; }
 };