diff --git a/README.md b/README.md
index b5a3b320..4d3bd121 100644
--- a/README.md
+++ b/README.md
@@ -164,7 +164,7 @@ An option name must start with a alphabetic character or underscore. For long op
 
 On a C++14 compiler, you can pass a callback function directly to `.add_flag`, while in C++11 mode you'll need to use `.add_flag_function` if you want a callback function. The function will be given the number of times the flag was passed. You can throw a relevant `CLI::ParseError` to signal a failure.
 
-On a compiler that supports C++17's `__has_include`, you can also use `std::optional`, `std::experimental::optional`, and `boost::optional` directly in an `add_option` call. See [CLI11 Internals] for information on how this was done and how you can add your own converters.
+On a compiler that supports C++17's `__has_include`, you can also use `std::optional`, `std::experimental::optional`, and `boost::optional` directly in an `add_option` call. If you don't have `__has_include`, you can define `CLI11_BOOST_OPTIONAL` before including CLI11 to manually add support for `boost::optional`. See [CLI11 Internals] for information on how this was done and how you can add your own converters.
 
 ### Example
 
diff --git a/include/CLI/Optional.hpp b/include/CLI/Optional.hpp
index 5b7d3e0b..1b2db6a2 100644
--- a/include/CLI/Optional.hpp
+++ b/include/CLI/Optional.hpp
@@ -29,7 +29,7 @@
 namespace CLI {
 
 #ifdef CLI11_STD_OPTIONAL
-template <typename T> std::istream &operator>>(std::istream &in, std::optional<T> &val) {
+template <typename T> std::istringstream &operator>>(std::istringstream &in, std::optional<T> &val) {
     T v;
     in >> v;
     val = v;
@@ -38,7 +38,7 @@ template <typename T> std::istream &operator>>(std::istream &in, std::optional<T
 #endif
 
 #ifdef CLI11_EXPERIMENTAL_OPTIONAL
-template <typename T> std::istream &operator>>(std::istream &in, std::experimental::optional<T> &val) {
+template <typename T> std::istringstream &operator>>(std::istringstream &in, std::experimental::optional<T> &val) {
     T v;
     in >> v;
     val = v;
diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index ab760247..4c191fcc 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -76,5 +76,9 @@ find_package(Boost 1.35)
 if(Boost_FOUND)
     target_link_libraries(informational PUBLIC Boost::boost)
     target_link_libraries(OptionalTest PUBLIC Boost::boost)
+
+    # Enforce Boost::Optional even if __has_include is missing on your compiler
+    target_compile_definitions(informational PUBLIC CLI11_BOOST_OPTIONAL)
+    target_compile_definitions(OptionalTest PUBLIC CLI11_BOOST_OPTIONAL)
 endif()