diff --git a/README.md b/README.md
index 0308fb9..cc5ec4a 100644
--- a/README.md
+++ b/README.md
@@ -26,7 +26,7 @@ Generators:
 | `basic_uuid_random_generator` | a function object that generates version 4 UUIDs using a pseudo-random number generator engine. |
 | `uuid_random_generator` | a `basic_uuid_random_generator` using the Marsenne Twister engine (`basic_uuid_random_generator<std::mt19937>`) |
 | `uuid_name_generator` | a function object that generates version 5, name-based UUIDs using SHA1 hashing. |
-| `uuid_system_generator` | a function object that generates new UUIDs using operating system resources (`CoCreateGuid` on Windows, `uuid_generate` on Linux, `CFUUIDCreate` on Mac) <br><br> **Note**: This is not part of the standard proposal. |
+| `uuid_system_generator` | a function object that generates new UUIDs using operating system resources (`CoCreateGuid` on Windows, `uuid_generate` on Linux, `CFUUIDCreate` on Mac) <br><br> **Note**: This is not part of the standard proposal. It is available only if the `UUID_SYSTEM_GENERATOR` macro is defined. |
 
 Utilities: