diff --git a/env_support/cmake/custom.cmake b/env_support/cmake/custom.cmake
index a9b79858d..5568e61dd 100644
--- a/env_support/cmake/custom.cmake
+++ b/env_support/cmake/custom.cmake
@@ -66,7 +66,7 @@ if("${INC_INSTALL_DIR}" STREQUAL "")
 endif()
 
 install(
-  DIRECTORY "${CMAKE_SOURCE_DIR}/src"
+  DIRECTORY "${CMAKE_CURRENT_SOURCE_DIR}/src"
   DESTINATION "${CMAKE_INSTALL_PREFIX}/${INC_INSTALL_DIR}/"
   FILES_MATCHING
   PATTERN "*.h")