diff --git a/CMake/GetGitRevisionDescription.cmake b/CMake/GetGitRevisionDescription.cmake
index a99610c43..b56558209 100644
--- a/CMake/GetGitRevisionDescription.cmake
+++ b/CMake/GetGitRevisionDescription.cmake
@@ -58,10 +58,12 @@ endfunction()
 function(get_git_head_revision _refspecvar _hashvar)
   _git_find_closest_git_dir("${CMAKE_CURRENT_SOURCE_DIR}" GIT_DIR)
 
-  file(RELATIVE_PATH _relative_to_source_dir "${CMAKE_CURRENT_SOURCE_DIR}" "${GIT_DIR}")
-  if (_relative_to_source_dir MATCHES "^[.][.]")
-    # We've gone above the CMake root dir.
-    set(GIT_DIR "")
+  if (NOT GIT_DIR STREQUAL "")
+    file(RELATIVE_PATH _relative_to_source_dir "${CMAKE_CURRENT_SOURCE_DIR}" "${GIT_DIR}")
+    if (_relative_to_source_dir MATCHES "^[.][.]")
+      # We've gone above the CMake root dir.
+      set(GIT_DIR "")
+    endif()
   endif()
   if (GIT_DIR STREQUAL "")
     set(${_refspecvar} "GITDIR-NOTFOUND" PARENT_SCOPE)