diff --git a/CMakeLists.txt b/CMakeLists.txt
index b9a49496222..642732ffe80 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -95,7 +95,11 @@ if (LLVM_ENABLE_PROJECTS_USED OR NOT LLVM_ENABLE_PROJECTS STREQUAL "")
       if(NOT EXISTS "${PROJ_DIR}" OR NOT IS_DIRECTORY "${PROJ_DIR}")
         message(FATAL_ERROR "LLVM_ENABLE_PROJECTS requests ${proj} but directory not found: ${PROJ_DIR}")
       endif()
-      set(LLVM_EXTERNAL_${upper_proj}_SOURCE_DIR "${CMAKE_CURRENT_SOURCE_DIR}/../${proj}" CACHE STRING "")
+      if( LLVM_EXTERNAL_${upper_proj}_SOURCE_DIR STREQUAL "" )
+        set(LLVM_EXTERNAL_${upper_proj}_SOURCE_DIR "${CMAKE_CURRENT_SOURCE_DIR}/../${proj}" CACHE PATH "" FORCE)
+      else()
+        set(LLVM_EXTERNAL_${upper_proj}_SOURCE_DIR "${CMAKE_CURRENT_SOURCE_DIR}/../${proj}" CACHE PATH "")
+      endif()
     elseif ("${proj}" IN_LIST LLVM_EXTERNAL_PROJECTS)
       message(STATUS "${proj} project is enabled")
       set(SHOULD_ENABLE_PROJECT TRUE)