diff --git a/CMakeLists.txt b/CMakeLists.txt
index c4a0634f0..b2d19524b 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -24,7 +24,7 @@ endif()
 #############################################################################
 
 # automatically parse the version number
-file(READ "${CMAKE_SOURCE_DIR}/Eigen/src/Core/util/Macros.h" _eigen_version_header)
+file(READ "${PROJECT_SOURCE_DIR}/Eigen/src/Core/util/Macros.h" _eigen_version_header)
 string(REGEX MATCH "define[ \t]+EIGEN_WORLD_VERSION[ \t]+([0-9]+)" _eigen_world_version_match "${_eigen_version_header}")
 set(EIGEN_WORLD_VERSION "${CMAKE_MATCH_1}")
 string(REGEX MATCH "define[ \t]+EIGEN_MAJOR_VERSION[ \t]+([0-9]+)" _eigen_major_version_match "${_eigen_version_header}")