diff --git a/CMakeLists.txt b/CMakeLists.txt
index 06761947b..7c4d8ad8c 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -8,9 +8,10 @@ project(nlohmann_json VERSION 3.9.1 LANGUAGES CXX)
 
 ##
 ## MAIN_PROJECT CHECK
+## determine if nlohmann_json is built as a subproject (using add_subdirectory) or if it is the main project
 ##
 set(MAIN_PROJECT OFF)
-if (CMAKE_SOURCE_DIR STREQUAL PROJECT_SOURCE_DIR)
+if (CMAKE_CURRENT_SOURCE_DIR STREQUAL CMAKE_SOURCE_DIR)
   set(MAIN_PROJECT ON)
 endif()