diff --git a/CMakeLists.txt b/CMakeLists.txt
index 43b49cefcbb4f5200d960aafa45e028807b44fe7..b45c5ee3773e1e4be8ee9fac40ad210caacd6d65 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -43,7 +43,7 @@ enable_testing()
 # Macro to add files to a var. Can even be used in subdirectories.
 # Source: http://stackoverflow.com/a/7049380/194894
 macro (add_files var)
-    file (RELATIVE_PATH _relPath "${CMAKE_SOURCE_DIR}" "${CMAKE_CURRENT_SOURCE_DIR}")
+    file (RELATIVE_PATH _relPath "${PROJECT_SOURCE_DIR}" "${CMAKE_CURRENT_SOURCE_DIR}")
     foreach (_src ${ARGN})
         if (_relPath)
             list (APPEND ${var} "${_relPath}/${_src}")