diff --git a/llvm/cmake/modules/AddLLVM.cmake b/llvm/cmake/modules/AddLLVM.cmake
--- a/llvm/cmake/modules/AddLLVM.cmake
+++ b/llvm/cmake/modules/AddLLVM.cmake
@@ -1500,7 +1500,12 @@
 sys.stdout.write(';'.join(relpath(p) for p in sys.argv[2].split(';')))"
     ${basedir}
     ${pathlist_escaped}
-    OUTPUT_VARIABLE pathlist_relative)
+    OUTPUT_VARIABLE pathlist_relative
+    ERROR_VARIABLE error
+    RESULT_VARIABLE result)
+  if (NOT result EQUAL 0)
+    message(FATAL_ERROR "make_paths_relative() failed due to error '${result}', with stderr\n${error}")
+  endif()
   set(${out_pathlist} "${pathlist_relative}" PARENT_SCOPE)
 endfunction()