diff --git a/CMakeLists.txt b/CMakeLists.txt
index a02cc8ce75..ef2b07a11f 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -18,15 +18,14 @@ project(libsecp256k1
 )
 
 if(CMAKE_VERSION VERSION_LESS 3.21)
-  get_directory_property(parent_directory PARENT_DIRECTORY)
-  if(parent_directory)
-    set(PROJECT_IS_TOP_LEVEL OFF CACHE INTERNAL "Emulates CMake 3.21+ behavior.")
-    set(${PROJECT_NAME}_IS_TOP_LEVEL OFF CACHE INTERNAL "Emulates CMake 3.21+ behavior.")
+  # Emulates CMake 3.21+ behavior.
+  if(CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR)
+    set(PROJECT_IS_TOP_LEVEL ON)
+    set(${PROJECT_NAME}_IS_TOP_LEVEL ON)
   else()
-    set(PROJECT_IS_TOP_LEVEL ON CACHE INTERNAL "Emulates CMake 3.21+ behavior.")
-    set(${PROJECT_NAME}_IS_TOP_LEVEL ON CACHE INTERNAL "Emulates CMake 3.21+ behavior.")
+    set(PROJECT_IS_TOP_LEVEL OFF)
+    set(${PROJECT_NAME}_IS_TOP_LEVEL OFF)
   endif()
-  unset(parent_directory)
 endif()
 
 # The library version is based on libtool versioning of the ABI. The set of