--- a/src/lib_proj.cmake
+++ b/src/lib_proj.cmake
@@ -292,7 +292,7 @@ source_group("Source Files\\Transformations"
 source_group("Source Files\\ISO19111"
   FILES ${SRC_LIBPROJ_ISO19111})
 
-include_directories(${CMAKE_SOURCE_DIR}/include)
+include_directories(${PROJ_SOURCE_DIR}/include)
 
 include_directories(${CMAKE_CURRENT_BINARY_DIR})
 source_group("CMake Files" FILES CMakeLists.txt)
