diff --git a/CMakeLists.txt b/CMakeLists.txt
index 93f7986a8..06761947b 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -9,6 +9,7 @@ project(nlohmann_json VERSION 3.9.1 LANGUAGES CXX)
 ##
 ## MAIN_PROJECT CHECK
 ##
+set(MAIN_PROJECT OFF)
 if (CMAKE_SOURCE_DIR STREQUAL PROJECT_SOURCE_DIR)
   set(MAIN_PROJECT ON)
 endif()