diff --git a/Help/variable/CMAKE_SYSTEM_NAME.rst b/Help/variable/CMAKE_SYSTEM_NAME.rst
index c3a42e5b41..fef53eecb9 100644
--- a/Help/variable/CMAKE_SYSTEM_NAME.rst
+++ b/Help/variable/CMAKE_SYSTEM_NAME.rst
@@ -4,6 +4,9 @@ CMAKE_SYSTEM_NAME
 The name of the operating system for which CMake is to build.
 See the :variable:`CMAKE_SYSTEM_VERSION` variable for the OS version.
 
+Note that ``CMAKE_SYSTEM_NAME`` is not set to anything by default when running
+in script mode, since it's not building anything.
+
 System Name for Host Builds
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^